Sandbox variable on Terminal Servers

I would like to create a variable on a terminal server (RDS) to point users sandboxes to a network share rather than user profile.

I want each users sandbox separate.

Will this work?

Variable name: THINSTALL_SANDBOX_DIR

Variable value: Z:\SANDBOXES\%USERNAME%

If not how could I do it.

Basically I am looking at page 16 of Peter Bjork's book here but it only refers to the variable pointing to one place for the sandbox, not lots of individual sandboxes.

Thanks.