No, that's a bit off. Maybe it's just the definition of the terms we're using, but if you have 28 initial and 35 total there can only be 7 degrees of advance not 33. If you have 28 degrees initial and add 33 degrees of mechanical you'd end up with 61 degrees total. Putting in a bushing to limit the mechanical advance will not change the initial timing. the only way you are going to change the initial is by turning the distributor.
Probably just confused with definitions of terms. Initial is the amount of timing advance (or retard) gained by the position of the distributor. When the initial is set the engine is at idle speed, vacuum advance is disconnected with vacuum source plugged and the distributor is turned to get the desired amount of advance. I like to start with 16-18 degrees. Some like more some like less.
Total is the amount of initial plus all the mechanical the distributor will gain. Again no vacuum advance is included in total. I know it doesn't make sense but that's how it's defined
Vacuum advance is the amount of advance the ignition will gain when vacuum is applied to the distributor's vacuum advance can. Vacuum will change with the position of the throttle plates in the carburetor. High vacuum with the throttles closed, low vacuum when the throttle is open.
The timing is changed as we drive in order to get the spark plugs to fire when the pistons are in the optimum position in the cylinder in order to make best use out of the cylinder pressure as the fuel air charge expands. As the engine rotates faster the ignition has to fire sooner to keep the maximum pressure and piston position constant. Pontiac did a pretty good job of determining the best firing points. Timing events only started to be set less than optimum when they started to change timing in an attempt to satisfy the smog rules.
I'd say a way to get your distributor to operate well is to put it back to the original configuration, if you still have all the original springs, weights etc. then adjusting basic timing. Once you get the basic timing done the mechanical advance can be messed with to change to rpm at which the advance peaks as well as the rate.
Try to start with a stock distributor. Disconnect the vacuum advance and plug the source. Set your initial (no mechanical advance, no vacuum advance) so the engine starts easily and idles smoothly. Try for a number between 10 and 18. Change it up and down by rotating the distributor until you get the idle and ease of starting you want. Then determine what your total advance is and at what rpm it is all in at. Rev it up until the distributor quits advancing and note the timing and rpm. it should be all in by 3000-3200 rpm. Don't worry if it's a bit up or down or what the actual amount of total is yet. Drive your car out to a remote quiet road and accelerate in second gear, if there is no sound of pinging adjust the initial up two degrees and try again. Keep going until you hear a ping then back the initial down two degrees. Check what the total is with this setting. If this total gives you too much or not enough initial, then you can play with the distributors mechanical advance springs weights etc to get you the total advance you found plus the initial you need.
Say you found the most total you want is 35 degrees and the best initial you found when you first started was 16 degrees, that's 19 degrees mechanical, but when you finished road testing the initial ended up at 18 degrees, that's only 17 degrees of advance. To keep the initial at 16 you would have to adjust the mechanical to get an extra 2 degrees of advance. Yours will most likely not be anywhere near these numbers I'm just picking them out of air for examples.
After all that is done you hook up the vacuum advance, I use manifold source, and re adjust the idle. that way you have the best initial for idle, the best total for power, and the vacuum advance for cruise economy and cooling.
Of course I'm not an expert, this is just my opinion.
