I smoked about a 18-20/day and have always been a really heavy smoker. It's all about how much you smoke your e-cig, I guess. The guy that got me into the ego-C's suggested the 24mg juice, and that 32mg would give him headaches, etc, and my wife, a slightly lighter smoker, says she's satisfied with the 24mg juice.
But, my wife smokes non-menthol and I smoke menthol, so instead of buying 2 bottles of flavor for every flavor (one menthol, one non-menthol), I asked them to give me some 100% menthol juice at 36mg. I noticed I preferred the more menthol flavoring, so on my latest flavor I'm trying from my local shop, I happened to get 32mg (Strawberry flavor) and actually prefer it, but for me it TASTES too strong. (50/50 pv/vg).
So what I've been doing lately is doing 50/50 with two flavors I like mixed, one 24mg and one 32mg, yeilding approx. 28mg/mL liquid which I really like (both flavor wise and nic wise).. I smoke 2.5mL of my "28mg" liquid per day. So that's pretty approx. but about double of what you are smoking -- and I smoked about double what you did in cigs. So I don't find that too much (but no expert, either).
There is a really great article on here about the safety, etc. of eCigs and their comparison to cigarettes that said that most vapers vape for an avg of 12 minutes at a time (vs 'analogs' aka cigarettes at 6 min).. So that made me feel better too.
Keep in mind at any point if you start feeling sick, dizzy, lightheaded, headache, nausea, or "weak".. Just STOP! That is "too much" and you'll know it.
Otherwise, just smoke until you're satisfied. I think you're pretty much the nail on your head, but once again, no expert here.