Commissioned to do a "(BIG) Piece of Work"

In November and December, I was commissioned to do two pieces which just about did me in.  They, of course, ended up taking much more time than I'd anticipated and made me very frustrated in the process. Oh, well. Live and Learn.  But, as a result, I think I've come up with a formula to determine how long a project will take me, and, hopefully, this will help me to make wise decisions as to whether I have the time to complete something or not.   Basically, my formula is:

Size of Piece (area in inches) x 75% (that's how much of a piece is generally the main subject, which takes the most time and effort) x .25 (I figure it takes me about 15 minutes or a quarter of an hour to workout one inch of work) = Total Hours (a piece will take)

I've used the formula on several pieces of art that I've completed and remember how long I spent working on, and it's pretty accurate.  It's too bad that I doubt anyone would pay me at least $10 an hour for my work--so I need to get much faster to make this hobby ever make much money!!  It's good thing it isn't my day job! (As a stay-at-home homeschooling mommy, that doesn't really pay much either!!!)

Here are some pictures one of my latest commissioned portraits, and some sketches I did for for the other: