how to use ready made drawer

[amateurSparky]

Hi
I brought some of http://www.screwfix.com/p/drawer-system-450mm/16338 ffor drawers for my work bench.

I have fixed the the guide rails in place where I want the drawers to be.
However am confused as to how do I know the width I need to make my base.
DO I just measure the inside width between the two wheels at the front?

Originally I measured the inside width between the two guide rails and found it to be around 5mm too big.

 

[foxhole]

A 450mm carcass with 20mm sides would leave 430mm space for the draw, you need to adjust according to thickness of your carcass.