The site computes the free item level inclusive of your current shopping cart.
So say your order history adds up to $400. If you want to see what you'd get with a $500 history, put $100 worth of stuff in your cart and move through the checkout. If you want to see at $1000, put $600 in, etc. You don't need to actually submit an order to see.
If I recall, the levels increase at $500 increments. $500, $1000, $1500, etc. The top level is "reef level" (at least I assume... I haven't been incremented beyond that yet and it's been a long time/lotta money). At that level you get $25-30 items like cleaner shrimp, yellow tang, mandarins, butterflys, etc.
What sucks is that once you increment to the next level, you can't get free stuff from a lower level, so if I don't want a $25 item, but I need something like a $10 crab, I can't get it.