It depends on which 1/2 HP shallow well jet pump you have.
If you have the J5S it probably won't work reliably at 40/60.
If you have the J5SH it will work at 40/60.
If you look at the nozzle and venturi part numbers in the table at the link you will see what the difference is. If you have the J5S you can substitute the nozzle and venturi for the J5SH and get the J5SH performance. The rest of the parts are the same.