I just called Les at LT-Racing to get clarification (he is a childhood best friend)....
The depth will depend on the model of shock, etc. If you follow the procedure outlined e.g. bottom out piston in reservoir, then install seal head/shock shaft in body, and extend shock shaft then the piston will be be 10-12 mm from bottomed out when you are done.
So - Measure the depth of the piston bottomed and after you install shock shaft and the difference should be 10-12mm..
jeff
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GasGas Riders Club