Center boot with Burton binding

I recently bought me a set of Burton bindings after using some Union Forces for about seven years. They are much more comfortable ofcourse but one thing I really liked about the Unions whas their adjustable heel cup. You could really center your boot in your bindings and above your board with them.

With the new Burtons this is just not possible. My heelside is hanging off my board more than my toeside and I'm already using the mounting holes closest to the heelside. Anything I could do about it?

I was thinking of moving the highbacks a little forward while making sure they still touch the heelcup but don't know if this is such a good idea.

Any advice please?

I've got a US11,5 boot and bindings in large. There is still about 1.5 cm (0.6 inch) more overhang on the heelside.

Normally it probably wouldn't be that much of a problem but since my board is only 25 cm at the waist I'm already pushing the limits ...