The rear changes about 2.8mm for every one 1mm difference from the stock 140mm between hole centers on the links. Does that help? I'm not quite sure what you seek.
That's a start, but it's only valid at a particular point in the travel. It's not a single value, but a curve. To do it right you really need to characterize the the curve throughout the whole range of travel. Either by a bunch of relative travel measurements (with a homemade telescoping measuring device in place of the shock) or by carefully measuring all the fixed and moving pivot and mounting points and modeling it in CAD software.
'07 DL650, '01 DR-Z400S, '99 SV650 (race bike), '80 GS1000S, '85 RZ350, '08 Ducati 848