[DesertMK4]

It is my understanding that the timing is set per specs on the engine, and more so due to the cam in that engine. Most engines have a base advance (usually 15-20 degrees) and a maximum advance (usually 30-40 degrees). A mechanical distributor should have springs and limiters to control the advance curve. With a vacuum advance there usually is some way to modify the curve but to a lesser degree than with a mechanical advance. The carbs have very little to do with ignition timing. Mostly its a matter of trial and error to set up your timing for your use of the car (race, street, etc.).
For street use a mechanical/vacuum advance may work better as a mechanical system advances in a more straight line whereas the vacuum advance responds to vacuum (load). One may "smooth" out the other.
Base timing is set at idle so what you idle at makes no difference. If vacuum advance only you must disconnect the vacuum to set the base. With mechanical/vacuum the vacuum port on the carb (Holley) does not pull a vacuum at idle.
Advance "all in" RPM is also engine dependent. Most Ford engines are "all in" at about 3500-4000 RPM.
It took me about 3 weeks to work mine out on a Ford crate engine. I ended up at base 18 degrees BTDC and topped out at 38 degrees. I have a mechanical/vacuum distributor.
Good luck with yours