IMO, and this topic gets debated regularly, it's not only tank dimensions that limit your stocking. many factors need to be looked at.
First off, how wide is your tank? my guess is it's not long enough to house a blue tang, but that's a separate issue.
As far as your stocking limit, I believe it also is a matter of your husbandry practices, filtration equipment,
LR volume, aquscape and the overall nature of the fish you currently have.
It's good to have lots of
LR for its effectiveness as a bio filter as well as it gives you options for aquascaping which helps you create "zones" within your tank. If you can aquascape it to minimize direct lines of sight from other areas within the tank, then you will most likely reduce aggressiveness from territorial issues amongst all fish.
Increasing your stock also means the water gets dirtier faster, so husbandry practices need to be evaluated. Having a GOOD skimmer will also help stretch the rule of thumb slightly too.
Include your dimensions of the cube on the next post and I'm sure many will give you their opinions on your limit.