I have an 85 Series Mallory HEI wired to a HyFire 6A box. I'm adjusting the vacuum advance using an allen wrench inserted into the nipple on the vacuum module. Is there a good rule of thumb to use when doing this? Like, 3 turns counter clockwise from fully tightened or is it just major trial and error?

Thanks!

Run a forum search...there are detailed instructions on VA and setting up centadvance etc

There is a lot of trial and error in addition to different parts affecting the personality of the motor as far as timing is concerned.

One thing I did was measure my manifold vacuum level at idle. I then used a hand vacuum pump connected to the advance can to advance the ignition with the engine running.. I tweaked the can to be full on at about 5" below my idle vacuum speed (12"Hg in my case). I also limited the stroke of the advance lever so I had about 22" of idle advance (base advance of about 12* + 10* vacuum). I'm sure your settings will be different, but the technique will help you get it tweaked. Timing light and a vacuum guage required (hand vacuum pump even better).

If you go to far below the idle..when u load the engine part throttle the VA doest cut out early enough , the curve is over advanced at that piont ping detonation even if not audable...and loss of power.
In most cases even 1" above idle vac is fine..because the engine fires, the VA kicks in, advances, the vac goes up...as it goes up it pulls on the VA more and so on..
The savest because of cam differences etc etc, it is best to put at idle vac with no va attached or 1 " below to 'make sure.

Unless one FULLY understands how the cent, how the VA works AND interrelates between the 2 do not mess with it...over advance, even at cruise kills slowly more engines than anything else...but other stuff usually gets blamed...including the engine builder. Engine builder hate ppl who tu tu with stuff they have little or no idea what they are doing.