It depends on the value you used for ERD. Spoke calculators usually call for a spoke which will end at the ERD entered. So if the ERD you used was based on the height to the tops of the nipples, then you have 1-2mm downside room. OTOH - if the ERD was based on the diameter to the bottom of the screwdriver slot, your downside room is more limited.

This is why I prefer to work with the rim's actual diameter at the nipple seat. I can then add the allowance for the desired height of the spoke into the nipple head to the calculated result and know exactly what I'm doing without depending on any assumptions. Based on this method, I consider 2mm nipple head engagement the minimum, with 3mm the ideal, and 4mm the usual maximum, though I'll sometimes go a bit longer if there's room for overrun.