1968 340 timing

I have been thinking about this,and doing mild research( I say mild because I dont have alot of time for research LOL!)
in my mind,running the vacuum advance on manifold vacuum makes sense. In this scenario, I could set my initial timing by the book at 5 btdc at idle because having the vac advance on manifold would put me at about 15 btdc,then add 20 for the mechanical for 35 total at 2500 3000 rpm. Then the rest is basically what is mentioned above by AJ/ form S. for power you want that advance always there untill total timing at 2500-3000 rpm, at which point the manifold vac drops off, the mechanical advance is already there. It makes sense now.. I will say though this is debatable and there are those even to this day that will say "Ported vac" is the way to go, ( i read in my research that ported was done as an emission control or something back in the 70s,maybe this is where that comes from, but we are talking performance here) but that dont make sense, the way I am seeing it is, on ported vac, by the time the vac advance kicks in, the mechanical advance is already there or behind by maybe a second. I also want to do the "distributor curving" with the X Y axis graph and be certain where my distributor advance curve is.