I guess that threshold depends on how long you will be immersed, which will vary from a few minutes to many hours if you are unfortunate enough to fall into the water miles from shore and lose contact with your boat to high winds, and no-one notices, as has happened all too often.
There are tables compiled that relate time immersed at a specific water temperature to loss of consciousness & death but I haven’t accessed them recently. (As I understand, some of the early work was highly unethical Nazi experimentation on unwilling concentration camp captives, so is uncited in the literature).
Mike’s advice seems spot on! Don’t bother to work out if you don’t actually need thermal protection around here – normally you need it!