20 seems a little high, but I don't think ridiculous...
Mine runs as low as 6 or 7, but I have a 70gpm sand filter (200 or 250 lb, I think) and my pump is rated at 75 gpm I think, so not much extra...
The 20 is the cumulative total of backpressure, mostly from the filter but also from the return lines; did you have to change any piping? and did you use the same size or bigger?