I would use similar but different approach. Remove the C and leading
zeros from the existing ID numbers, leaving only the real numbers.

Then highlight the entire range and apply a custom number format of:

C00000

That will cause 35 to appear as C00035...but the raw number of 35 is
still visible.

Then in the "next number" box at the top, just let is show the Raw
number to type next...36, not C00036. When they type in 36, it will
appear as C00036.

Now your MAX formula will work.


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JBeaucaire
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