If i back out the limbs down to 60#'s how much am i gonna change the a to a or brace height--less poundage=shorter a to a?

The change in draw weight will probably not change the axle to axle length significantly on most bows. When I say significantly I mean by more than a fraction of an inch. Same goes for brace height. A bow's brace height is generally measured with the bow at peak draw weight. By going to the opposite end of the draw weight range for any given bow the brace height normally does increase but, again, not that significantly.

Thanks PA,The reason i asked about this was siiting here thinking(that's usually a bad thing)about the problems some people were having with some of the MATHEWS bows with the roller guard and the string not staying on the rollers--it was a thought.