I may have confused you a bit when I started talking about amp ratings, but that was mainly to tie everything back into Ohm's Law.
When we pick how many ohms we want our coils to be, what we're actually doing is setting the amperage rating of the coils which determines the amount of current to let through. Steam engine uses a resistance formula to reverse-calculate the watts based on the current rating we choose for our coils.
So, it sounds like you mainly need some help figuring out Steam Engine. For starters, I highly recommend you play around with the Wire Wizard, and plug in some different values so you can see the relationships between the gauge, windings, heat capacity, heat flux and current per volt. This is the hardest part to figure out, and four years later I still have a lot of trial and error when I build coils.
Just select your inner coil diameter and amount of wraps (8 is default, but 6.5 is a good number). Then select single wire and Kanthal A1 30g should be the default. The display area on the left will show you the attributes of your selection, and this is what you want to pay attention to as you play around. As you change the number of windings, gauge, inner diameter, etc, these numbers will all change.
I've found, through my trials and tribulations, that I prefer a heat flux in the 10mW/mm2, and a heat capacity in the 35mJ/K range. The heat flux determines how hot the vape is, and your heat capacity determines the ramp-up time. Basically it's a warm vape with a medium-quick ramp up period. However this is all personal preference and your preferred vape will probably be different.
Of course I also build for a mechanical squonker (a Reo Grand) so I'm completely at the mercy of Ohm's Law, while you have some adjustability.
Your max wattage will be determined by your batteries' max amperage. Two 20A batteries in parallel can handle 40A max, which would be 168 watts (4.2 x 40 = 168) riding the ragged edge.
I'm not sure if steam engine has a reverse calculator, however it does give you a "current per volt" rating on the Wire Wizard page. So if you want to figure it out yourself, the formula would be:
Voltage x (voltage x current per volt) = max watts.
For instance you have a 26ga coil with 7.5 wraps and a 2.5mm ID, giving a current per volt of 1.12A. That would look like:
4.2 x (4.2 x 1.12) = 4.2 x (4.7) = 19.76 watts @ 4.7 amps
Hope that helps clear up some of the confusion and gets you a bit further along.