It's kind of annoying being unable to maximize a mist tank use, so how about the following:

Allow overfilling up to 150ish ME with a tank.

Decay any overfill at the same rate as the daily allowance, making each 800 second me tick push everyone's tank one point towards 100.

Only allow 100 ME in any one action to avoid crafting exploits.

Personally, I think a better promo item would be a book of 10 elevator tickets (2 for mini tanks) also usable as a res ticket as long as the price isn't too high.