By 50-60% I would guess you mean in the middle (or so) of the gauge? Ether way 20% sounds low and can lead to problems, ie; engine can't get up to operating temp and it will run rich.
do a google on engine not warming up.

I think you can remove the rad. cap and watch to see if the coolant is circulating when the engine is still cold. That would indicate a stuck open thermostat, crazier things have happened.

If the 20% range is normal then carry on!!
Maybe someone should answer that one...

1994 Montero SR
Divide Colorado