I think all these questions were hashed out, but just in case...

Formation construction options are determined during army selection, including free ones. (Epic UK has their exception for SMs.)

A formation can drop units before the game without penalty. No BMs.

Formation cost are determined during army selection, so the BTS is determined at that point as well.

In the highly unlikely event of a GT tie-breaker situation where the difference in unit count makes a difference in whether the formation is below half strength, count based on the units that were actually deployed. (If your'e dropping units, you're obviously doing it for some sort of strategic advantage so there ought to be at least some sort of token drawback.)

Are you saying consider the size of the formation based on deployed units not purchased? Personally I'd do it the other way and check using the purchased unit count.

I would do it on purchased unit count to. Possibly a hold over from playing games with 'break points'.

However I can see situations where it would be 'odd' - if you had a large formation and dropped half during deployment it would be impossible to get them to half strength without wiping them out!

For that reason I would do half of destroyed units initially deployed to get the half strength.

So you would comare the number of destroyed units against the number deployed to dtermine whether it is understrength, but consider the original purchase cost as the value of the formation for these purposes.

In the same way that the formation is still the BTS even though it left several units behind at the start of the game - seems OK to me

I think Nealhunt has retracted that paragraph!

Okay, I didn't write that well and I didn't think about the option of counting "left behind" units as destroyed.

Should it be...

1) Count units on-board for half-strength, based on the original formation. If you left behind 2 units of a 10 unit formation, for example, you'd be at "half strength" after only 3 kills. Effectively, this counts left-behind units as destroyed.

2) Count units destroyed, based on the original formation. For a 10 unit formation, you'd need to lose 6 units be be below half strength, so if you left 2 off board, that's 6 of the 8 on-board units to count as below half strength. This effectively counts left-behind units as always functional.
(Obviously, this could be seen as giving a boost and was what I was thinking about when I responded.)

3) Count units based on the deployed formation. If you left behind 2 units in a 10 unit formation, you would determine half strength as if the formation were 8 units. Left-behind units are ignored.

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Anything but #3 gets into some freaky stuff with Marines leaving behind transports because of the number swings.

Tacs + Razorback + Hunter + Rhinos gives you the option of garrisoning if you drop a 25 point unit and 3 free units. However, your unit count goes from 11 to 7, which is a huge swing. #1 counts them as half destroyed with only 2 kills. #2 requires 6 out of 7 units to count for points (very bad, imho). #3 would require 4 kills.

Also, back in the dark ages of Epic playtesting when this first came up it was in the context of Ork Warbands and the question was whether you could get the Big/Uge discount even if you didn't have enough models to field. A KoS + 7 Skorchas bought as Extras is 375 points, or you could get a Big KoS for 350 points and then leave one unit behind. There was really only an intent to field a 15-unit formation and it seems sort of against the spirit of the "you can drop units" ruling to penalize or benefit the player for it.

I've only ever done #3.