Hank, 

Are you refering to a loaded fixed bullet cartridge, or?

Here's one way...

After you've charged your case with powder, tap it lightly to level the powder. Lightly place a slip-fitting wood dowel, aluminum or brass rod on top of the powder and make a mark on your dowel even with the case mouth. This is your zero compression mark for this particular brand, granulation, and powder charge weight. 

Remove your dowel and compress the powder, keeping in mind that you don't want to crush the grains or distort the case.   

Place your dowel back on top of the powder and make a second mark on the dowel, again even with the case mouth. Remove your dowel and measure the distance between the two marks to obtain the compression measurement. 

You'll generally want to start with light compression with your first batch of test loads and see how well they group. Then slightly vary the amount of compression applied with each successive batch, until you find the amount that gives you the best results. 

If you decide to change the amount of powder you're using (or the granulation, or the brand), you go through the process again. 

After you drop tube your powder charge into the case, Insert a wad and push it down against the powder charge till it stops. Measure from the edge of the brass case down too the wad with a depth gauge of some type..I use a 6in. Caliper...and take a reading. Then after you compress the powder down to the amount needed for the Over All Length of the Cartridge, Measure down too the wad again..This will give you the amount of compression for that load.

Terry

Thanks for your answers, no for question #2 , how far do you compress black  powder for breech seating

Like Frank I throw then tap the case with a brass rod. Push a wad on top to keep it from spilling then insert behind the bullet.

I gave up on compression as not practical breech seating not necessary for fixed. I do settle the powder by tapping.

Boats