I am no expert on this, but maybe this explanation may help.
Total timing is WITHOUT the vacuum advance connected - so this must be disconnected and the hose end plugged off. So if you want 36-38 degrees total advance, this is a combination of where you set your timing mark on the balancer at idle speed, 12 - 14 degrees. The distributor has the weights & springs that add additional timing advance as the RPM's of your engine rise - this is called your advance curve. The rate at which the advance curve kicks in is based on how much tension the springs offer that are attached to the weights. There are light, medium, and high tension springs to "tune" your distributor's mechanical advance. These springs are purchased in kit form from aftermarket suppliers. Your factory distributor is set-up by the factory and should operate as the factory specifications call for. Any modifications to your engine can change the factory specs and this is where you play around with the springs/tension to adjust the timing curve that best suits your engine -and sometimes the gas/octane you are using.
The built-in mechanical advance in the distributor for a Pontiac should be around 24 degrees. Add your initial timing at idle, 6 1/2 degrees, and you get 30 1/2 total ( you won't get 36-38 total degrees setting the balancer at 6 1/2). To get 36-38 as you say you want, then you will have to set your idle around 12-14 degrees - which may be too much and cause hard starts, or worse, pinging/detonation which will damage your engine. So, if you set your balancer timing at 8 degrees, add distributor timing of 24 degrees, you get a total advance of 32, 10 will get you 34, 12 - 36, and 14 -38. You will have to play around with this.
Now the total advance can be made to be all in at 2,500 - 3,500 RPM's depending on the tension of the springs you use and how much movement your weights actually have. Aftermarket distributors offer more adjustments to include weight stops to dial in the mechanical advance. Factory distributors don't have this and fabrication of some kind is needed, but you have to know what you are doing.
Once you have your timing set, THEN you connect your vacuum advance and this can add up to 10-15 more degrees, but only when the car is cruising under part throttle.
Check out this website: https://www.msdperformance.com/support/set_your_timing/