Couple of issues.
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If you measure from center of BB to top of saddle, did you also account for changes like Crank Length, Shoe and Pedal Stack Height, Saddle Fore-Aft Position?
- Every one of those can be and is likely different from a road bike to MTB.
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Don’t assume that what is right for road is also right for MTB. Plenty of reasons to have subtle differences in saddle height, fore-aft saddle position and bar drop. You might use them as a starting point, but using them without consideration to the huge differences in use, is not likely the best approach.