I remember the theory but the the maths I forget, I'd also forgotten about steam engine. As the voltage in the battery decreases the mod pulls more and more amps from the battery to maintain the wattage. Hence you need to know how many amps you are pulling when reach your personal battery voltage minimum, and make sure the cell can support that current draw,
I've used 25Rs before and found them to be seriously lacking.
I've been using Molicel P28s for a long time. I use a single battery mod with a pair of them in constant rotation, and they don't last more than a year which I suppose is ok. The P30B is supposadly a 30amp cell, I need half of that, surely I could scarifice some of that amp draw for more amp hours no?
The Sony VTC6 looks to be a good match for me.
No one has commented on the eVic mod, and I would surprised if they did. The last one I bought was new old stock and that was about three years ago. Probably got no chance of finding one now.
Perhaps I could ask for suggestions on a modern replacement?
Single battery regulated mod thats good up to 50 watts, no temp control just wattage. Needs to have a really firm battery door, loose wobbly doors drive me nuts. Tidy, small simple, tough, I love to drop a mod.