I AM CONCERNED THAT APLYING THIN SET UNDER MY CEMENT BOArD MY DOOR WILL NOT OPEN. hOW MUCH THICKER WILL IT MAKE THE TILE
If you want the installation to fail quickly then don't use the thinset under the cement board. The thinset (under the board) represents about an elevation change of about 3/32" if applied in the correct manner. The thinset is there to 100% fill voids created when the fasteners are installed into the cement board.
Using ALL CAPS isn't necessary here, we all can hear you just fine.
You can use a product called Ditra made by Schluter. The build up of the tile will be much less. Go to www.schluter.com for more info. I have installed several thousand sq.ft. of tile over Ditra over the last 10 years with no issues.