A dead screen is not automatically a failed screen
The little unit has disappeared, your opponent is pleased with the shooting, and the expensive thing behind it is still alive. So far, that might be a perfectly respectable exchange. The uncomfortable bit comes a turn later, when the gap is still open and your army has nothing suitable left to put in it.
Warhammer 40K screening is often explained as putting something cheap in the way. That is the beginning of the decision, not the whole of it. At a tournament, the useful question is what that sacrifice buys you, and whether you can spend the time it creates.
Games Workshop made the distinction in its 1 October commentary following the confirmed 30 September balance update. Its Tyranid example uses Termagants to protect an Exocrine from arriving Chaos Terminators: stronger enemy weapons do not automatically make the screen pointless. That is a dated official example, not a claim that a new update arrived this morning. The practical lesson I would take into an event is to judge protection by the options it removes, rather than by how many screening models survive.
Start with what you cannot afford to lose yet
Before spreading models across a lane, decide what you are protecting and for how long. A gun-beast that needs another useful shooting phase, a melee unit waiting for a worthwhile commitment and the only remaining unit capable of reaching a late objective are different problems. They should not automatically get the same screen.
In a hypothetical Tyranid game, protecting the Exocrine might be worth giving up a small unit if that preserves the shooting needed to stop the next push. It becomes a poor bargain if the creature has no worthwhile shot, the rest of the army cannot use the space, and you have sacrificed your last flexible scoring piece simply because it was nearest.
Name the benefit before making the move. An extra turn of useful shooting is a benefit. Making the enemy take the long route can be a benefit. Hoping they get distracted by the small models is more of a personality assessment.
Remove the first line before you place the second
During practice, physically lift your first screen off the board. Do not leave it there while admiring how safe everything looks. Now check the position you will actually have if your opponent clears it before committing the next threat.
Look at when the hole opens. Shooting away a screen is not the same sequence as clearing it in the Fight phase. Those timings can leave different charge, consolidation and next-turn movement possibilities. Read the relevant sequence and the opponent's abilities; a tidy-looking gap is not a universal safety distance.
Your second line might be another small unit, a protected position the valuable unit can reach, or a counterattack that makes entering the gap costly. It does not have to be another straight row of bodies. What matters is having an answer that still exists after the first exchange, and can legally get where it is needed. Two screens stacked together can also give the attacker two useful targets rather than two separate problems.
Check the enemy's route, not just your ruler
The Core Rules separate movement, charging, consolidation and arriving from strategic reserves. Treat them as separate checks. Keeping a reserve arrival away from one spot does not prove that an enemy already on the table cannot reach it, or that every special arrival ability follows the same restriction.
Use the actual bases and models. Check the shortest legal route, the space around each end of the screen, terrain permissions, coherency and where the attacker can finish. Ask about unusual movement or arrival rules before relying on the formation. Terrain can complicate the route without making the unit behind it untouchable.
This is why I would avoid learning one magic screening distance and applying it to every matchup. A formation that inconveniences a large ground vehicle may present a different problem to infantry, a flying model or a unit with an explicit exception. The event's current rules and the actual board decide whether your idea works.
Leave your own army a way through
A wonderfully secure position is not much use if your own damage dealers cannot leave it. Before finishing the screen, rehearse the next friendly move as well. Can the valuable unit reach its firing position? Can the counterattack get around the screen? Does the transport have a usable exit route, and can its passengers be set up legally when required?
Then look at the mission. Do not spend every mobile unit on protection and discover that nobody is available to do the scoring. Equally, do not send the replacement screen on a speculative errand if losing that protection exposes the unit your whole next turn depends on. These are competing jobs; list-building needs to leave you a genuine choice between them.
Our map-practice Field Note is a useful companion here. This is not a formation to memorise in isolation. Rehearse it on the layout your event uses, with objectives, terrain and an opponent's plausible routes included.
Practise the failure, not the photograph
For the next practice session, pick one important unit and one likely attacking package. Set up the protection, then run the position with the first screen removed. Try the obvious route and the awkward one. If the important unit is exposed, change the position or the resources assigned to it rather than relying on the opponent overlooking the opening.
Before taking the list to an event, check its current costs, the relevant rules updates and the organiser's cutoff. The official downloads page checked on 16 October labels the Munitorum Field Manual updated 14 October; that label alone does not establish which individual costs changed, and this guide makes no numerical points claims.
Finish the rehearsal by writing down what takes over when the first unit goes. If there is no answer, you have found something useful before round one. Better to discover that on a quiet practice evening than while your opponent is reaching for the charge dice.