I understand the directions on the starting load being 80% of the powder space for a given cartridge but what I'm wondering about is what is the effect of different bullet weights. For example what differences would there be with a 458 Win Mag cartridge if both were loaded to 80% but one is with a 300 grain bullet while the other was with a 500 gr bullet/

What has been taken into account is that they have calculated that for any given bullet (profile/weight) at it's seating depth, the powder charge weight will be within safe parameters.
What you see (when the round is fired) is the alteration in pressure and velocity for the change in bullet weight/profile.
Very often heavier bullets are more efficient (internal ballistics) when using Trail Boss, especially when loading to subsonic levels.

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Despite the stupid European law banning many of these powders I continue to use Trail Boss.

Following the guidance on the TB table, mark the bullet depth on the case using your bullet of choice.

This volume becomes the maximum load. Work your load from there as RW already posted.john

I had no idea you could use Trail Boss this way! About to have some fun with my 22-250.