The ideal battery for a pulse x 90w mod would be something that could handle 30A such as the efest 20700. Then you know your with in limits all the way to the full potential of the mod
The chances that you have 20% power loss in your mod is not very likely the average of modern mods is only about 10 percent. Which at 40w it means your only pulling about 13.8A from your battery and that's only if your vaping while your battery is absolutely flat. Most mods cut off at higher...