The transitions actually happen at:
35/36mm Sand
91/92mm Sand / Sandy Loam
146/147mm Sandy Loam
202/203mm Sandy Loam / Loam
258/259mm Loam
313/314mm Laom / Silty Loam
369/370mm Silty Loam
425/426mm Silty Loam / Silt
480/481mm Silt

these values are all calculated with the 20mm bias, using the excel equation

=CEILING.MATH(J19*500/8.99)-20

This info should probably also be on the player's field-guide

The comment is intended behavior. The issue is the incorrect sign on the operation below. However at this point changing it will also constitute a world generation breaking change (as in harsh borders) which is less than optimal, so it might be works-as-not-intended at this point / for a while.

I don't see any real reason this or these values need to be present in the field guide. As per our guideline

• It SHOULD contain all information that someone would NEED to play TFC, to a reasonable degree of competence.

The exact rainfall thresholds that govern soil transitions (which are intentionally fuzzy and a bit hidden / trying to be blended) don't really fall into that at all.

I believe the bias is actually correct, if it were reversed then it would cause the silt-only region never to spawn (unintuitively)

No, it is not. I wrote it, I know the intent. The implementation is wrong.