Thanks for your replies.
You are least bouyant at max depth at the beginning of the dive when you have most gas and most compression of the wetsuit, and most bouyant at the end when you are doing your shallow safety stop. Its the difference between these two stages. I see three factors- air difference, wetsuit compression difference and error factor from overweighting.
The air is knowable - say 200 bar difference X 15 lt = under 4 kilos.
Ive no theoretical idea or practical memory (too long ago) of the bouyancy difference of a 5mm wetsuit at say 6 bar to 1.5 bar.
Error is likely to be greater when using a set-up for the first time - so this should be allowed for (recently Ive only used 15lt steel cylinders with a drysuit).
Hence it was good to hear that in practice one of you found no problem using the travel wing with a 15lt cylinder- ----?at depth and with a 5mm wetsuit?
Thanks