Please reread the earlier posts by SkyeFire and I for the explanation of the equation itself.
The numbers all come from the KR3 data sheet. As previously mentioned, the 100 is the A2 to A1 offset for the KR3. The 265 is the distance from A2 to A3 for a KR3. And the 270 is the distance from A3 to A5 (the wrist root) for a KR3.