First I would be sure your gauge was accurate with 90psi reading, if so its a good thing you reduced the psi. 90 would be way too high if you have 90psi at all your water taps if this is a residential system,you would be having problems with your toilet flush, dishwasher valves, sink faucets, etc.
The greatest impact on water usage would be adjusting the amount of time the sprinklers are on, yes a lower pressure would reduce the amount of water passing thru the head, but the % I am not sure about.