I've made a simple Excel calculator to convert between dosage of KNO3, KH2PO4, K2SO4, and MgSO4 and the resulting amount of the nutrients NO3, PO4, K, and Mg. The main innovation is that you can specify each nutrient individually. If you want a lot of K but not much NO3, have at it.
Unlike the PPS-Pro calculators, this spreadsheet is not locked down; feel free to alter it. (I'm releasing it under CC BY-SA 4.0
Given your tank volume, the total amount of solution you're making, and the amount of that solution you plan to add to the tank, it'll either give you the ppm of each nutrient for a given fert mix, or tell you what fertilizers to mix to produce the given nutrient levels.
A screenshot may be more enlightening. (Never mind the odd nutrient levels; I'm working off a big NO3 surplus.) The yellow cells are inputs; the red text are outputs.
I think it'd be nice to have a bit more consistent discussion of fertilizer levels in terms of ppm per day/week of each nutrient. I know it took me a long while to get a sense of what the distinction between PPS and EI is, for instance. The rota.la calculator is great, but it doesn't help you control the balance between the three nutrients. For instance, I wrote this calculator to figure out a low-NO3 dosage for my over-fed tank.
I make no promises of user-friendliness. If you have ideas for better documentation or design, be my guest.
Download from MEGA - v.20140911