Instruction Skill Options

[PraetorGradivus]

Let's say I'm improving my pistols skill from 7 to 8.
The table says I got to spend 8 weeks training.
I go to Buffalo Bill Cody who has a Charisma of 6 and an Instruction of 3.
Does Cody roll once (and assuming avg roll) reduce it to 7 weeks, 4days?
Or does he roll once each day and on avg reduce the training by three days so that I'm finished in 2 weeks?

And, yes, I know, page 105: If the player is able to find an instructor to train him and help him perfect his techniques, the time it takes improve his skill is reduced by 25 percent (round down).
But this and the rules under the instruction skill don't actually mesh and assumes every instructor is equally skilled. I'd rather have instructor's who vary, just as in real life.