How much volume is lost after compacting 3/4" crushed stone?
When something calls for x" of compacted y, you typically calculate the amount of y prior to compacting. 

So if something calls for a 4" compacted base, order enough to cover your area with 4". Then compact. 

(Actually, order to cover 4", but the only lay down half, compact that, then lay down the other half, then compact that.)

In other words, you don't typically order the stone to allow for compaction. You order the stone based on the volume needed prior to compaction. 

UPDATE:

If you're just asking about the rate of compaction 3/4 minus has, the answer is...actually, that's a tough one. There seems to be a lot of opinion, but no specific engineering spec that I can find. General googling seems to imply you'd be in the 20% loss due to compaction range, but I can't find any hard data to back that up.

Keep in mind that there's also issues of your soil base compacting as well. 

So based on that limited info, I'd suggest ordering 20% over. Keep in mind that this may be a trial-and-error process that may not be worth the headache. If you can't get 20% compaction, then you're left with extra crushed stone that may or may not be easy to get rid of.