Okay, here's a screenshot of the software. The first picture is the stock tables as they come from MSD. If you take and add the values at any one point, you'll get your total advance. So say for cruising at 2000 rpm and light loads (high manifold vacuum, which equals low pressure on the top graph), you'd have 22 degrees of "mechanical timing" (bottom graph) plus 12 degrees of vacuum advance. If you punch it WOT at that state, your manifold pressure should go to around 14 psi (typical atmospheric pressure), which would give you 22 degrees of "mechanical timing" (still 2000 rpm on bottom chart) plus -8 degrees from the 14 psi on the top chart for a total of 22-8 or 14 degrees.
Now, if you modify the tables to look more like a typical distributor, that would mean your vacuum advance goes to 0 at atmospheric pressure (~14 psi). The second picture is an example of this. However, if you don't modify the "mechanical" advance, you might be pushing the limits of the engine. In the same example above (WOT at 2000 rpm), you're now pushing 22 degrees of "mechanical" timing and 0 degrees of vacuum. Not unreasonable numbers, but the mechanical chart climbs all the way to 27 degrees, which most agree is way past what the new hemis like to see. To combat this, you can lower the "mechanical" timing of the bottom chart. In the second picture I've pulled 7 degrees from the table, so now when you go WOT at 2000 RPM, you still have 0 vacuum advance, but you're mechancial is capped at 14 degrees, just like the stock table from the first picture.
The two pictures are almost equivalent from a total timing perspective, but you'd have to add a lot more vacuum advance at lower pressures to keep them more identical. If you were to make the vacuum advance graph a straight line that just slopes from some starting value to zero you'd be on the right path I believe.
