I've measure on my side,
front/wall corner: 20 3/4 front/center corner: 20 1/4
rear/wall corner: 19 3/4 rear/center corner: 19 1/2
has been redone by the previous owner so I can't be sure all has been put back all at the same place. I think I'll fabricate two wooden support and give a little put on the cabinet to bring it higher up (1/8 to 1/4 inch), this will insure the roof is well supported.
Thanks all !