To get my Average Feet of Head, I followed step 7 at
Having calculated GPM (see math above), next figure out the Average Feet of Head for the pool pump i.e. average amount of feet from where the suction lines are (2 skimmers & main drain) back to where the pool pump is.
The lengths of each line are:
Skimmer 1: 5' + 10' = 15'
Skimmer 2: 5' + 40' = 45' (spa)
Main Drain: 10' + 10' = 20'
To get the average Feet of Head take the three suction line lengths and divide by three. 15' + 45' + 20' = 80' / 3 = 26.6 average Feet of Head
The reason given for me needing 1 or 1.5 hp is because I have the built-in over-flow spa with a separate blower and if i want to use the spa to it's full potential, I need 1 to 1.5 hp.
As for filters, what is DE and how does filter size relate to pump or pool size?