I find the easiest way to do a premixed fertilizer is to first decide how much liquid I want to dose at a time. That could be a syringe full, a cap full, or something like that. For example, in my case I wanted to dose 1 ounce per dose. Next, based on the size of the bottle, figure out how many of those doses are in the bottle. In my case it is a 16 oz bottle, so I have 16 doses in the full bottle. Next I calculate how many tsp of KH2PO4 I need per dose (for example). If it is 1/8 tsp, I multiply that by 16, and that is how much KH2PO4 I put in the bottle with 16 ounces of water. Now, my one ounce dose will contain the right amount of fert for one dose.
I mix KNO3 and KH2PO4 together in one bottle, and CSM+B and MgSO4 together in another bottle. Since I dose every day, dosing both mixes each day, my mixtures are pretty weak, so they dissolve easily. And, because they are so weak I don't worry about dosing iron and phosphate on the same day.