Ok....... maybe we can word it another way. When you mix your Peters 15-5-15, how much Peters 15-5-15 do you add to the amount of water you use?
So two quantities will be required here ------ 1) the amount of fresh-water you use ..... and 2) the amount of Peters 15-5-15 that you add to that water.
At the moment, we're having an issue of not knowing the final concentration of Peters you use for the watering mix. So everybody has been trying to find out. But you don't provide answers for some reason. Normally, that would lead to my second question ----- as in 'why?' (but I won't ask that - as we just want to help you and your
orchid out
).