Here is how I do it, with no issue at all after building many, many wheels.
ERD measurement tool making
1. Cut the spokes to 200mm. If you cut them to 200.6mm, or whatever, file them down until they are EXACTLY 200mm. This is not hard.
2. Screw on any standard nipple (12mm, 16mm, pink, blue, neon black, whatever), making sure to screw it down EXACTLY to where the top of the spoke is completely flush with the slot cut into the top of the nipple; this is all that matters. The length of the nipple is irrelevant, as this is just how long the rectangular part is that the spoke key engages with; the heads will be the same inside the rim.
3. When you are happy you have it in the right position, make a visual, mental note of where the top of the spoke sits, then open your super glue pot. Unscrew the spoke slightly, drip in some superglue down into the nipple, then quickly screw the spoke back up to the perfect, flush position you had it at before. Leave it for a few minutes, to set fully.
1. Cut little bits of white electrical tape, and number then 1 - 24 (or however many holes you have on your rim). Put these on the rim at each hole.
2. Get a notebook and pen. Write the following; 1-13=, 2-14=, 3-15=, 4-16=, 5-17=, 6-18=, 7-19=, 8-20=, 9-21=, 10-22=, 11-23=, 12-24=
3. Get a steel ruler.
4. Put the two spoke/nipple sets through opposing holes in the rim, and lay them onto the steel ruler. Make a note of the length (200+200+whatever the measurement on the ruler is). Write this into your notebook at whatever nipple hole crossing you used.
5. Move the spoke-nipple sets to the next opposing nipple hole crossing round, measure again and record. Do this all the way round, then average. That is your ERD.
Then, use one of the online spoke length calculators to get your spoke length. I prefer the WheelProUK one.
You can even set up a spreadsheet in Google Drive and record your various rim measurements for later use, if you want to do things extremely thoroughly. I have a single spreadsheet in Drive, which I have all of my wheel builds on, which I record ERD, spoke tension round the wheel, tyre inflation / fitting spoke tension reduction, etc. like the massive nerd I truly am.