You should be able to get a good enough estimate with the spokes in the wheel.

I'll just lay a new spoke up next to the existing spokes, and guess where the nipple will stop.

There was someone complaining about removing consecutive spokes throwing his wheel out of true. I don't know if any damage was done, but you're probably better off doing a spoke on the opposite side.

Still, that seems to be extra work.

As FBinNY mentioned, for replacing a couple of spokes, it doesn't have to be exact. If you have a single walled rim, the worst thing would be filing down a bit of spoke sticking out.

If you're building a new wheel, it is nice to start with good spokes of the right length, but the ones that came out aren't necessarily the perfect length either, especially if changing the dishing. A lot of spokes stop short of the nipple head, which can put stress on the nipple, so a perfect length will go through the nipple head.