Yes, 38 would be about the limit, but it's only a "rule of thumb".
Nothing set in concrete.
Stance/body positioning could help,
Flattening a sail could help or hurt (depends on how flat the water is, and how fast your top speed
So, lot's of "factors"!
The design of the fin can have a significant effect as well.
The Lessacher Duo (Speed and Weed fins with an assymetrical design) can be run much smaller
due to having a bit more area.
Hope this helps,