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Personally I used trial and error. As my processing workflow improves so does my practical magnitude limit which is around +13.0 to +13.3 for my dslr tripod AP. Basically after I do a good processing (rather than just quick and dirty) I can compare my shot to star charts and lookup the vmags of the dots I'm confident are signal in my shot rather than alignment of noise. Doesn't take long really to do and only need a handful to check. I can then be confident about deciding to chase a target or not as I just wont get anything beyond my range since my processing is at my limit for the data. I usually use 500 dslr subs so I'm about the limit for dinishing returns reach already, plus diminishing wakefullness and unblocked skyview. I dont know the theoretical limit of my gear and should look into that maybe, not that I can afford to replace anything. But this way I have a number to work against to make good use of my time chasing targets I know I can bring out in the shot. Damned annoying pluto its just hovering out of my reach, even still I've done sessions where its in view and I fully processed but couldn't find it in the signal at all, confirming my estimatation is practical.
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