take whatever grams you want /lets take 5g/in 500ml water.PO4 in KH2PO4 is 69,8%
so [(5x0,698)x1000] / 500=6,98mg PO4 in each one ml of this solution.
If you want to add 1mg/l PO4 in 10g/38,7l/ water(for example)- (1x38,7)/6,98=5,54ml of this solution.
hope this will help