How about: hover over a blank spot in your rack ...and a popup tells you how many blank HP are there. ..
-- pescatore
+1

Also, in the summary at the bottom of your rack,
- show the width in hp
- show the name of the deepest module

For me using a rather shallow skiff it's a constant nuisance when planning on using a module,
learning late in the decision process, it's too deep for my case.
Speaking of which it would be helpful in the process of 'manufacturer approving' to make sure
power requirements and depth of the module is given.