Start at the recommended(even though it sounds high) and then work it down(or up) based on plant response and coverage needs. You really don't want it more than 12"-15" at the absolute closest, so the 20" they said really it's not too far off. Good LED's, even average ones nowadays, don't need/want to be close, they are just as powerful as hps(didn't say it covers the same)... light is light and if it's too powerful it will harm plants. 18" is a good standard height in led's right now for this point in technology.
Vega, I know you are a CFL guy...look into the cree globes if you haven't already. 3x the output of cfl's with 1/3 the watts. I just tested the par of globes vs cfl last night and was super impressed.