I have a bass that seems to have come with a slope in the heel of the neck, from the factory. A friend of mine levelled all the frets but could only take the last few so far before they were too low to go further. I took a pair of calipers to the neck & noticed that around the 21st fret the heel is 1.5mm thinner than at the 24th fret, despite the pocket being the same depth all along. I don't have the money to have the frets pulled, board sanded down & refretted. What would be the better way to go about fixing this? Should I sand the bottom of the heel down so it's an equal thickness, or should I do what I've done temporarily & shim the back of the heel up the 1.5mm needed to equal them out?