How to set timing for initial start up on new build?

It is "not quite" correct

The rotor only selects the cylinder, it does not "set" the timing.

If you are running points, once you get the rotor "on" the no1 cylinder, move the distributor retarded, then advance until the points open. Use a test lamp, etc

If you are running breakerless, set the distributor so the reluctor is in the center of the core. This is not 100% accurate

Once you "get the hang" of this you can actually turn on the ignition power, move the dist "retarded" and then "move" it towards advanced until you generate a spark. Do this back and forth several times, trying to slow down rotating your distributor, and stop the rotation right at the spark point. With practice, you can get this within a few degrees

ALSO you can set initial timing "on the starter" while cranking