Sounds like you might have to calibrate the zoom curve since it’s non-linear.
Make a table with a number of samples. Example: .1 = 25mm, .2 = 38mm, etc.
Use this as a form of Lookup table and lerp between the closest steps or curve fit the data so you simply end up with 100 steps (or whatever resolution you need) so that a value of .25 provides you 31mm output.
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