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I have a question about stocking. I have read the recommendations for calculating the limit of fish per gal., but I am wondering where inverts fall into the equation. How do I determine when enough is enough with crabs, star fish, shrimp, etc... plus the fish?

LM, the ultimate judge is your water quality.  Test your water a few times after purchasing a fish and see what it says.  The most incredible filter in the world wont be able to help an over stocked tank.  I really don't go by the method of fish per gallon.  I try to judge how much space the occupants I currently have use and determine if they're going to be upset if somebody else comes in.  Usually you'll have small fights break out between fish before you exceed your filter's capabilities, even if you only have live rock and no filter.  As for inverts, most are useful in lowering levels but of course do add something to the tank as well although in very small quantities.  My only advice is to observe your inhabitants, if they look healthy and aren't fighting you can probably safely add someone else to the tank. 

You're quite right in relating the number of fish one could keep to the filtration of the tank. However, it is not the only consideration. One should also look at the size and temperament of the different species that you want to keep together. Some species are very territorial, and will become aggressive or stressed due to "overcrowding" long before the filtration & water quality becomes the limiting factor.

Maximum expected (or rather, possible) water temperature is another important consideration. For people living in colder climates, where the tank's water must be heated throughout the year, it is not a problem. For someone like myself, who stays in a hot summer region, it becomes a real problem. My tank's temperature regularly rises to 30-31 °C (86°F-88°F). As the water's oxygen carrying capacity is severely diminished at these high temperatures, an even "slightly" overcrowded tank would cause severe stress.

The type of set-up should also play an important role in determining maximum number of fish. A "fish only" set-up can happily exist at a nitrate level of 20ppm, while a "reef" set-up should have a maximum nitrate level of 5ppm. One could thus (obviously) keep more "fish per gallon" in a "fish only" tank than in a reef tank. So, one should consider all the factors involved before deciding on a "rule of thumb". In a mature, cool (75°F), well filtered fish only tank your thumb could be rather longer than in a hot, Berlin type reef tank.