I never have done an ignition setting by your way with calculation of the TDC, as on newer Bultaco the piston pin sits not in the center of the piston also not recommended, (they changed the piston-pin position on later Bul's to reduce the tilt effect of the ultra short hub engines for the 350cc Sherpa's).
As you wrote I also adjust at first the gap between the points. Then mount back the rotor.
Now I search the TDC, for measuring the TDC i use a gauge, like this:
Its a standard gauge with an extension pin that sits in a nylon shaft that you srew in the cylinder where the plug should be. The inner diameter of the shaft is slightly smaller as the shaft of the gauge so you can adjust the gauge to zero.
(The washer is only for rough adjustment.)
Now I mount a beeper to the ignition wiring, when it beeps I now know where the ignition point actually is, a lamp works too but you can only focus on one thing with your eyes so I like the peeper solution more.
When the timing is wrong / not in the right distance before TDC I now take off the rotor and adjust the stator new.
The beeper, (original Spanish Motoplat), or a standard Multimeter:
Then I mount the rotor back again and do a second test, when it's right it's ok if not I have to readjust the ignition again, there is sadly no better way to adjust the stator. Normally I have to do two adjustments and the ignition timing is spot on, (You have to get used to this procedure, btw. even stated in the Bultaco users manual).
When this is done I check the gap of the points again, if they are still in right spec's the job if done, if not you sadly have to do the hole procedure again, luckily this hasn't happend to me yet.
Your timing specs are very forwarded to the one's I know from my model?