I assume then that you did not measure actual stroke of the slave ?, if the book says the clutch needs 12.5 mm minimum, this is the minimum not the maximum, it just means no more push will create any more release, you won't over stroke the slave as it has a stop, unless it is a centre push ?.
Assuming it is on the std arm with a external slave cylinder, and you know the bore sizes and stroke(and therefore CC) of the cylinders you can do a few things to limit if needed the stroke of the slave, if needed make a spacer, this can be just washers, and place these on the push rod inside the slave so the piston travel is limited by the new mechanical stop as opposed to butting the end stop.
You can also alter to pivot point on the pedal, the way I do it, it is adjustable so not only can you select the height of the pedal but also the pressure ratio, using a longer stroke for easier pedal but same fluid movement as a shorter but harder stroke, just reset the pivot point at the pedal, to lose travel of course set the connection point for the master cyl push rod closer to the pivot shaft for the pedal.
The bearing on the clutch arm is about 1/2 way along, so 12.5mm is going to be a LOT more movement when added to the length of the arm where the clutch slave attaches, typically 30-35mm.
tabetha
tabetha