Rear wheel rim holes (continued)

Another picture of the rear wheel with the pattern of the holes for the spokes:

I made the pattern like this: I used a strip of paper which I put around the rim. This gave me the correct circumference. I removed the strip and divided it into 20 equal parts and then put the strip back on the rim and transferred the marks from the paper to the rim.

First I was thinking to use a piece of yarn to do this, but yarn stretches and paper does not.

The bolts I would like to use as nipples are these (I found a couple in my drawer - I will need to buy the other 75 soon enough):

As you can see, the niple standing up already has the M3 thread cut on the inside for the spoke.