How much does it 'change'? Is it repeatable? Does it go back to the 'right' place if you loosen the clamp? Can you just account for how much timing will change with the initial position of the disty and then have it move to where you want it to be once it's tightened?
If it's repeatable, and not jumping around, I would suggest just using that 'pre-tightening' offset method and then tighten to where the timing moves to the 'right' place and use it that way a while. Then go back with the light and see if it's still in the 'right' place. If so, close the hood and go have a beer. You're done.