More info. A basic 10-10-10 fertilizer with N adjusted to 100 will provide 100/44/83 elemental ppm of N/P/K
264.17 is the PPM of a gram or ml in a gallon, so N (first number of guaranteed analysis) / 100 * 264.17 * grams/ml = ppm of N. 1 gram of 10-10-10 in a gallon: 10/100*264.17*1 = 26.42 ppm N. 4 grams of 10-10-10 in a gallon = 105.7 ppm N. There are other ways to do it but that's what works for me. It's the same for P and K except for P the result is multiplied by .44 and for K the result is multiplied by .83. So with 10-10-10 and N adjusted to 100 ppm instead of 100/100/100 you end up with 100/44/83.
A basic fertilizer like that can produce a good crop, no doubt about it. I've seen it happen, dude pulling dank and good yield using Miracle Gro 10-10-10 from start to finish.
There is evidence higher P and/or K can be helpful during bloom, not just with our crop but all flowering and fruiting plants. That's why you see tomato fertilizer with something like 12-15-30, 18-18-21, 4-7-10 or 10-5-15... all tomato specific fertilizers. If we normalize the N values to 10 we come up with 10-12.5-25, 10-10-16.66, 10-17.5-25, 10-5-15. If we use those value to create a solution with 100 PPM N we get these values:
100/55/250
100/44/207
100/77/250
100/22/124
Full swing bloom I'm running 100/80/180 so I'm probably hitting the upper levels of useful P at 80 ppm in bloom and could probably bump up the K a little from 180. Like I said, it's a work in progress, but all this goes to show there's probably not a critical point at which PK should be boosted. Do it early or wait til you see buds forming, or don't do it at all provided you're running adequate PK to begin with, it will be okay either way. One guy I was reading about is using 100/50/150 from start to finish, swears by it... or maybe it's just easy but he's happy with the results.