Help with ignition timing

I ran the MP electronic distributor in mine and it has 25 degrees on the distributor. I like to run 13 initial and 38 total (vacuum disconnected while checking). I found this chart on another forum recently:

dist. degrees X 2 + initial= total


dist. degrees / slot size
6.............. .340
7............... .355
8............... .375
9............... .390
10.............. .405
11.5 ........... .420
12.............. .435
13.............. .445
14.............. .460
15.............. .475
16.............. .490
17.............. .505
18.............. .520