Qwas dogs rock. Some of my other blog postings show uses for calibrating the position and square of the main MFT fence. Qwas himself (some call him Steve) has some YouTube videos to show them being used for squared cuts in the field of the MFT, etc.
Awhile ago, I ordered a Starrett adhesive tape (metric/imperial) from Lee-Valley and put it on my MFT fence. Naturally, this needs to be easily calibrated to the cut line. What I did is this:
Measure it; since I use a metric/imperial tape on the fence, I measured it in millimeters since I could easily eyeball down to a third of a millimeter. Write it down on the block.
Now that the alignment block is ready, calibrating the fence is trivial: drop the flag stop and put it on the measurement of the block (142mm in my case), push the MDF against the dog and the fence... slide the fence over until the flag stop abuts the MDF. Now go build a walnut keeper box with handcut dovetails to store this precious alignment tool.