jem, it's really tough to tell someone else how cold they will feel in a particular temp of water. Everyone's personal thermostat is different.
We are Canadian, but currently live in the SE United States. Since our background is cold, low vis diving, drysuits are 2nd nature to us.
That being said, we both only own two types of exposure suits: a full 1/8" (about 3mm) wetsuit and a DUI 350TLS trilam drysuit.
Hypothermic me wears the drysuit in anything lower than 75F (about 24C). I just vary what I wear under the suit: a t-shirt and bicycle leggings for "warmer" water; various layers (usually sweats) for "colder" water.
I routinely dive various FL springs (about 20C) with a long sleeved t-shirt or a sweatshirt & bicycle leggings under my drysuit. Of course I also like to wear a little 1/8" "dive beanie" to keep the cranium warm.
Meanwhile, we were in the Red Sea at the end of March. Everyone on board the boat wore 5-7mm suits, hoods & gloves execpt us; we were wearing our drysuits. The water was a bit less than 20C, and I was comfortable wearing a long sleeved sweat shirt along with my leggings.
Does this help at all?
~SubMariner~