How to calculate how much boiling water is needed to get the pool’s temperature up:

(T1*L1 + T2*L2) / (L1+L2) = Tm

T1= Temperature of heated Water
T2= Temperatur of poolwater
L1= Amount of heated water
L2= Amount of pool water
Tm= Temperature after mixing 

Notes: 
– Assume the water has 90°, you won’t be able to get 100° hot water in
– This will give you a rough idea of how much hot water is needed. If you end up with, let’s say 100 liters that will be needed but you only can fill in 10 liters at a time, you can at least expect a significant jump in temperature after those 10.