I am not a expert but I made a couple of pieces of cuir bouilli. (made a lot of mistake at first, now it's starting to look like I want)
If your your water is just a little colder your leather will still harden but will not shrink dramaticly.
the hoter the water the harde the leather, but do not ''cook'' your leather.
carving will not be as clear,
do not use water base tint (eco flow from tandy) before you bouille leather.
Like rcsaddles said testing is always to ways to go