The heatmat covers 3/4 of the viv?! This is far too large, it should only cover about a third of the floor, this gives a proper gradient down the viv.
To set the thermostat up set it to 95 and place the probe at the hot end. Then use a reliable thermometer to check the temps in the viv - don't just rely on what the dial on the stat says, you may have to fiddle with it a little to get your temps spot on.
is the rule not 2/3 for lizards generally? and 1/3 for snakes?
it really depends on species in as much as if it was a beardie u wouldnt set the probe on the warm end as the warm end can be very warm, urd set it near the cool side so the cool end didnt go to high but warm end can be high as you like within reason.