A) the length & width of the desk. Unless the front & back edges are one peice going from left to right over the *leg area* the support is not equal over the bottom edge of the tank.
B) how thick the top of the desk is.
I just converted a console style living room table to hold my 100g and ended up putting extra 4x4s at the corners and along the back edge to make sure the top would not sag in any way putting stress on the tank frame and causing a possible seam leak. Last thing I want is 100g of water on my floor or having to figure out temporary homes for the fish while I drain and reseal a leaky seam.