I'm building a wheel where all the calculations show a 264.7mm spoke length - with 14mm nipples

I have a spare set of 36 x 263mm butted Sapim spokes that I would like to use.

My gut feeling is that it will be OK. Any views please, before I start the build?

I think it will be perfectly fine. I actually try to go 1 mm shorter than the spocalc for my wheels.

Agree that it should be fine. I don't think most spokes are supplied in point-millimetre sizes, anyway, so you would be rounding down or up (usually down). The calc should have the end of the spoke at least a millimetre from the head of the nipple, so there is a little bit of scope to play with on this one.

If the spokes have been used before, check any deformation at the elbow and the "mushroom" on the end. Any bends along a spoke should be OK when it's tensioned.

It doesn't take that long to lace up a wheel to the point that you can tell if your spokes are long enough.

I want the spoke end to reach the bottom of the screwdriver slot in the nipple, for max strength.

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.