Setting the pressure at 50 PSI is the same as 115' of head. So, you need to add the 115' to the 20' up and add a little friction loss for the distance to the total head the pump needs to supply. I would get a pump that could do the flow needed at about 150' of head. Also, with a jet pump to get 50 PSI you need a 40/60 pressure switch to be about to set the CSV at 50 PSI constant, and then the pump needs to be able to build a max pressure of 70 to work with a 40/60 switch.as for the actual pump, i am planning to have it outside the tank. it won't be quite at the level of the bottom of the tank, but will be well below the 'full' level. It looks like folks here like Gould pumps, so i thought i'd look at what they make. It has to push the water 20 ft up and 40 ft out, and deliver at better than 5gpm (a single hard running faucet worth) at that height. It won't be pulling the water more than a couple feet up, at most. I'm planning to set the pressure at 50.