Not an expert but with blackpowder and different length bullets I change my powder charge to keep the compression about the same.  You have to experiment with each bullet individually to see what charge and amount of compression, if any, that your rifle likes.  A load with little or no compression will be shoot different then one with more compression and that is where you need to load enough combinations to test which one is most accurate with that bullet and powder make.  Tom

Forgot to add that I use 1F, 2F Swiss and Goex 1F (yes I still have some) in 8.15 x 46R, 32-40, 9.5 x47R and 8 x 57R. The 9.5 and the 8.15 I compress slightly when seating projectiles (I can hear the powder crush) the others are breach seated. Will be buying 1 1/2 Swiss when I run out of the other Swiss powders that I have as this seems to be the preference in our top shooters down here in Adelaide South Australia.

start with .030 compression and keep adding a grain of powder till you get best accuracy   don't compress with the bullet!  art  long time bp shooter

Thank you all for your feedback, had a brief range session on Tuesday, and found  I appeared to be getting tighter groups with more compressed loads, this came about through using two different brands of cases (starline and W-W), the WW cases have  less  internal capacity, and to get the correct seating depth (just touching lands) had to compress about an eighth of an inch more, suddenly got tighter groups, have now  loaded more W-W and will go to range on Monday, and test further. Was compressing with projectile but now do not, using compressing die, fire formed  cases and finger seated projectiles. Do bell case mouth fractionally to ensue  that no damage to projectile when seating. Slowly getting there, thanks again for all your input/feedback, d.