When you do the rough calcs to decide whether a power plant is worth the tile, don't forget to include production from your Initial Colony tile. That ususally adds a 'factory' or two to your total, depending on the tech you have & whether it's your homeworld.
I always just count that as 1. To simplify.
If you have a +10% production bonus, the break-even point for Anti-Matter is 5.5. Round up to 6. Quantum break-even is basically 4.
If you're Drengin, it makes even less sense. The first 20% PP (e.g. Devil's Forge) is the same as Anti-Matter, but the second, the break-even point is 6.5. But I would actually round that down to 6, due to the influence cost of adding another Slave Canyon.
If you're Iconian w/ that 50% Interstellar Refinery (or whatever), stacking an Anti-Matter won't help until you have 8 factories. 7, with Quantum.
(1+planetary bonus+PP) x = (1+planetary bonus)(x+1)
Solve for x, that gives your break-even point. You could also plug in base ip for a more accurate result, but it's not worth it.