OK - so this wont be of interest to most people but having done the work I may as well post it as it could help someone in the future. I struggle to understand and remember things if I cant visualise them, if I'd been born in the 90s no doubt this would have a name and I'd have pills for it but I wasn't, I'm just 'a bit slow'.
All protaganist legend variations of the GENIO POPVLI ROMANI type Follis during the 1st Tetrarchy (293-305). Dates of know issues in (brackets)
The areas they controlled - not important for my point but a nice map 
The 2nd Tetrarchy and beyond (305 Onwards) after Maximian retires (briefly, then returns) and Galerius becomes Augustus in the East. I've not included the other protaganists as they are unimportant as far as the Maximian / Galerius clashes are concerned.
The legend clashes are shown in
orange in the two tables and it is these coins that are probably the most mis-attributed of all of the approx 4000 Genio Pop issues. The coins farthest from 305 in date are the easiest to attribute style wise. As Doug states above it gets progressively harder as we get closer to the 305 cross over where the titulature changes. One piece of advice I would give is don't attempt to attribute from pictures on sites you don't fully trust as I have found more often than not they are wrong or inaccurate.
The best two sites I'm aware of - certainly for the London issues:
http://jp29.org/plnhome.htmhttp://www.numisology.com/index.htm