I think I am making this more complicated then it needs to be.
As soon as you make enough passive income to cover all of your expenses your are FI. This we all know and are working towards in our own way.
Getting to early retirement through ERE, to me, means optimizing your expenses so they are a small fraction of your income. Save and invest the surplus income wisely. Eventually get to the point where the income from investments can safely provide for all your optimized expenses. Learn to live happily on the reduced expense foot print which is more of a journey then something you could change overnight.
I think you need to get an accurate picture of where you are now in terms of income vs expenses. So using an average of the expense portion of your joint transfer is a good idea. Cost allocation is actually a whole chapter or two in most accounting text books. Just make sure to recalculate the average if things change, which they should if you reduce expenses in an ERE way.
Once you know what your ongoing expense requirements really are you'll likely need to start cutting expenses to get into ERE territory. Generally this means creative expense reductions from the big 3 expenses drivers (Housing, Food and Transportation). However in your case that might be different but Mint will give you a decent category breakdown.
Mr Money Mustache explains the math better then I can but you could probably retire in 16 years or so if you saved 50% of your take home income. If you get to a 75% rate and it's sustainable you could retire in about 7 years starting from scratch.
You just need to sit down and create an
accurate line by line breakdown of all your current annual income minus annual expenses. Go through your banking history, use Mint, make estimates based on what you know is going to happen. Divide the annual numbers by 12 to get the monthly income minus expenses, this way you don't have to worry about amortization it will be done automatically. Then you'll know your real savings rate and can use the ERE math to your advantage. If you use incorrect expenses the math will give you incorrect solutions (garbage in garbage out as the saying goes). When you make changes just recalculate the annual or monthly amounts using a spreadsheet, Bob's your Uncle.