What are the times by which Nasus should have certain Stack sizes?

I've been picking Nasus back up the last few games, and keep missing Q stacks by slivers, and in general don't seem to be building them up very quickly. What are some good X stacks by Y gametime numbers to look for? 300 by 20 minutes is the generally preferred metric to aim for, no? So far I seem to be hitting 200 stacks or so at 20 minutes. Game I just finished I had about 450 stacks at the end of a 40 minute game, so I seem to be hitting about 10 stacks a minute or so. That feels slow to me, and I know part of it is because I'll go to grab one and flub it by a sliver of HP. Having to watch out for aggressive lane opponents hurts my stacking too.

http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2351736597/37223151?tab=builds My most recent game.

Using flat AD reds and quints with flat armor yellows and flat mr blues, 12/0/18 Masteries, Grasp Keystone. I normally max Wither before Spirit Fire, but I was against a Yasuo and wanted to zone him off a bit.