Honing guide measurements?

Millimeters = equals degrees with the most basic vise type of guide, bought mine at a Rockler store. The instructions only give the following:
Chisel projection: 30mm = 30 / 40mm = 25 Plane iron projection: 38mm = 30 / 50mm = 25
Time for 1 more for the micro bevel of either, can anyone tell me how many less millimeters to subtract from the projections?
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Alex
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It ought to me accurate enuf to interpolate: -2 mm for chisels and -2.5 mm for plane irons should give you a 1degree change.
Walt C

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Really cool, thanks Walt!
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Alex
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