If those are the purple Efest 2600maH "40A" batteries, Mooch tested them and says that they are only re-wrapped 20 Amp batteries:

https://www.facebook.com/1636157550....1073741828.1636157550007158/1674040996218813
The formula I recommend (conservative engineer here) for calculating the battery current is:

I=Power/Vmin-cutoff/efficiency, where Vmin-cutoff is your mods minimum cuttoff voltage (typically around 3.2V, but for your Alien 220, djlsbVapes tested it and found it to be around 2.8V per cell (very low!):

http://www.liquidjuicevaping.com/d2884c.video
For efficiency I use 0.85%, and with your two batteries in series, the minimum voltage SHOULD be 3.2V x 2 = 6.4V, and

I = 75W/6.4V/0.85=13.8 Amps draw from your two batteries in series.

If you use the 2.8V value as the cutoff per djlsbVapes testing however, you get 15.8 Amps.

I don't really use a rule of thumb, I use the calculations above. If I had to pick a maximum power for a 20A battery as a rule of thumb, it would be 50W, which results in (for a single batter mod):

I=50W/3.2V cutoff/0.85 efficiency = 18.4 Amps, which keeps me under the 20 Amp continuous rating where I would rather be

So for a 2 battery series mod, 100 W max will keep you at 18.4 Amps.as long as you don't go below 3.2V battery voltage per cell.