Cluelessfish:
I run a similar system with a rubbermaid type sump on my 30 and I have not had a problem at all with the slight angle. In fact it is quite incredible. Coming from a more expensive liter meter for top off that quit on me, this product is quite amazing and much more accurate as it adjusts for changes in evaporation (something that changes in this humid climate daily). It keeps the system exactly where I have it set and has numerous redundant safety features that make it outstanding for a top off system. I am absolutley in love with this system.
Only problem you will have is with the instructions. They are simply inadequate and confusing, but if you follow the pictures, you can get through it. The rails make for a nice system that holds where you put them, but the instructions are not the friendliest for putting it together.
Best regards,
Tim