If you plan to do much of this type of work I would suggest that you obtain a good steel rule and an inexpensive pair of calipers; a tape measure is a little crude for this kind of work. They are inexpensive and will last forever.

If you are stuck with a tape, don't use the tape's hook for a starting point; instead start on, say, the 10mm mark and then measure from there. The hook is movably attached to accommodate both inside and outside measurements so is non-ideal for either.