I know this will be more difficult to measure, but your pedal travel should be stroking the master cylinder ~29mm to get the ~16mm at the slave.
your 13mm at the slave implies only ~23mm at the MC.
Is your MC the original ~19mm BORE? with the slave bore of ~25mm? vendors do sell a long stroke slave which has a smaller bore. (smaller bore gives MORE stroke from the same MC)
If the 12mm slave stroke is all you can get, then you might consider changing the length of the lever arm to the ToB?
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