Any new divers reading this should be aware that tables are based on MAXIMUM depth, and using average depth with the RDP or other recreational dive tables (with very specific exceptions) is violating the model and therefore cannot be predicted to be safe. The models were not validated using average depth. There are those of us using a strategy involving average depth to manage our decompression status, but we are using other tables and accepting a lack of documented validation to do so.
bradells, as I said, I kind of build a profile in my head. My average recreational dive is about 60 minutes long. Shore dives involve starting in very shallow water, swimming deep, and returning, and are often rather check-marked in shape. I have a sense for how much time I spent in the deep segment of the dive, as a proportion of that 60 minutes -- it's usually about a third. The other two-thirds are spent in the shallow phase, because of the tail of the check mark. If I build the picture in my head, I can draw a line across it that looks like the average depth, and I have been surprised at how well that works. It fails if the profile is egregiously sawtoothed, but I almost never do that except in caves, and to be honest, I often let the computer tell me the average in a cave with a lot of ups and downs.