Chain stay length has really nothing to do with saddle fore/aft. It's saddle tip relative to a vertical line coming up through the center of the bottom bracket you should be concerned with. Either make it the same as it was or on an XS I'd probably put your saddle tip pretty damn close to even with that line for a noseless saddle, and well in front of it if for some odd reason you are still using a traditional saddle.

An alternative method would be to measure the actual seat tube angle from horizontal. Again use the center of the BB as one point, and the middle of the usable portion of the saddle rails for the 2nd. Set that at 80 degree using a simple smart phone app held flush with a standard level.