What does the 30/45/60/90 min refer to? Time between sets? How many sets?
I do what feels good that day. Start with a set of 30 then four sets of 40, finishing with a set of 30. The only set that maxed was that last set of 40 and the final set of 30. Total 180 push-ups, not bad for an old fart.
Stew Smith's workout info looks like good gouge.
For example, say your max in 2 minutes is 60 (that is what you do first thing Monday morning).
Every 60 minutes you will do one set of 18 pushups (30% of your 60 max). You do that until you go to sleep at night. Typically, I would cut it off at 10:00 p.m.
Tuesday, you do 30 pushups every 60 minutes (50% of 60).
Wednesday, you get the idea.
Some days aren't too bad, others are killers. However, the good thing is it really only takes a minute to jump down, push out a set, and get back to working or whatever currently occupies your time. There will be times when you simply can't jump down and push out that set ever 30 minutes, but for the most part, try to stick to the schedule and you will be amazed at the results. I think I went from 58 to 72 after one week.