This is what I found works for me after reading on this board and experimenting for a while.

Unhook your vacuum advance pod. Hook a vacuum gauge to manifold vacuum, and advance the timing at idle until the needle on the vacuum gauge quits climbing. You may need to adjust your idle lower while doing this.

Calculate how much mechanical advance you need the distributor to add by using that initial timing number minus approximately 34 to 36 total. Weld up the advance slots in the distributor just enough to limit the mechanical advance to the number you calculated. A chain saw file works very well to file and adjust the slots once welded.(i.e. 18 initial - 34 total = 16 degrees of mechanical advance.)

Determine what your normal cruise rpm is on the highway. Be sure you use advance springs that allow it to be all in a few hundred rpm below that.

Run a vacuum advance pod hooked to manifold vacuum. They are adjustable in both the amount of vacuum that initiates their advance and in the amount of advance. This takes some experimentation, but start on the lower end of the scale. I found on longer highway trips that I can get a gas knock that doesn't occur around town or on short highway trips. I suspect it's heat building up in the head in places a bit more than it does on short trips. I had to use less vacuum advance to compensate for this.

My slant six likes 24 initial and 35 total. I also run a short throw vacuum advance pod hooked to manifold vacuum. Total timing at idle with it hooked up is in the mid 40's. The timing drops back to 24 when the throttle is stabbed at low rpm, until the mechanical advance kicks in. No pinging, way more power & torque and better economy when set-up like this.

My 493 inch RB likes nearly the same, but a tad less initial and all in at 1800 rpm. It has no vacuum advance. That may change one day.