While advice has been given, is it usefull. In my opinion, No! Most of the riding will be done on the slide cutaway and needle jet and little on the main jet.
Giving advice without knowing the gas level in the bowl, condition of the carb body, size of the slide cutaway and condition of the needle jet (as little as a few tenths of a thousandths will change the jettng where the bike is run the most) is a waste of time.
I would be interested in the condition of the body - worn bodies lean the mixture at lower throttle openings.
Condition and size of the needle jet - worn needle jets make the bike run very rich at lower throttle openings.
Height of gas in float bowl
etc. Now thats the advice I would be looking for.
Now most of you have never checked or replaced your needle jet, but it is singly the most used (and abused) jet in a AMAL
While you want to have the proper main jet, unless you are running at throttle positions well above 1/2 throttle (3/4 -Full), the main jet has no appreciable effect at the throttle openings typically used for street riding. If you choose to run your bike with straight pipes shifting at 3/4 plus throttle all the time you would probably be arrested for the noise you would make.
As far as guessing the main jet size when changing exhausts, compression, etc look up the stock jet and get one two or three sizes larger. Go from their.
Dan, if you are seeing performance changes at lower throttle openings when you change your main jet something is radically wrong. You should be able to take the main jet right out of the carb and the bike should run right up to 1/2 throttle with few rich symptoms.
Often I find with straight pipes you end up with a smaller main jet, but let the bike tell you what it needs.