How much does vertical pipe count against the 50' limit

Hi all; Planning stages on my second rmh. My question, if a 8" system can push apx. 50' with each 90 counting as a 5' deduction ,when the pipe turns up to go vertical do you keep measuring and deducting off of your 50' limit,or do you stop measuring at that point as the heat will rise on it's own ? My shop has tall ceiling and i would like to have as much mass on the floor as possible if i have to count the 16' going up to the ceiling I won't get much left to go in the mass. Thanks tom

http://en.wikipedia.org/wiki/Stack_effect

http://www.engineeringtoolbox.com/major-loss-ducts-tubes-d_459.html