Hi,
One thing I haven't been able to tell from the posts here, is weather the leather is being pulled off of the backing,,, or if the backing is being pulled off the leather. If you are pulling the leather off of the backing,,, you will have all kinds of stretching, & distortion. Always pull the backing off of the leather!
Now is a good time to review Bob Parks casing proceedure!!
My proceedure is: apply 2 crossing layers of good packing tape to the back of the leather, then apply rubber cement to the backing, & the packing tape. When dry/tacky stick them together, using the slicker, on the backing side, to make sure they are stuck good. After the tooling is all done,,, & dry,,, lay the leather face down on some clean paper towels, & start peeling the backing off,,, making sure that the leather remains flat against the paper towels. Once the backing is off, do the same with the packing tape.
Things that will cause distortion: peeling the leather off of the backing,,, removing the backing while the leather is wet,,, bending the leather during the removal process. Doing any of these will reduce the tooling depth also.
Note about using packing tape: If the item is going to be lined,,, there is no need for the tape! Also, if you are going to be using tape,,, get the good stuff. The cheap stuff, doesn't stick good enough,,, plus the cheap stuff stretches. If the tape stretches, that defeats the reason for using it.
When using "Plexyglass" it should be at least 1/4" thick. Any thinner, & you'll get lots of cupping. I have some "Plexyglass" that I've used that is several years old, & is still nice & flat. Also another nice thing about using a backing is that while tooling, if you want to cover a portion, with some plastic, to help keep in the moisture, you can tape it to the back to hold it in place, while tooling the rest.
I hope this is of some help,,, Ed the"BearMan"