TDEV cache usage used to be somewhat of a straight up equation, but is now a much more complex algorithm that allows for much greater efficiency. So it is not really something that can be answered here. Contact your EMC account team as they will have tools that can help you with cache sizing and usage.
I believe when we create tdev and do not bind to any pool, it should be in Global memory. What will be the size of unbound tdev (eg 100GB)?  Thanks, Shailender
Tdev size