Now I have to figure out the cost of a mixed commute based on getting 6 days out of a 12 ride pass (round-trip, remember). The 12-ride costs $57. So now I get to play with a larger formula involving a term based on the number of days I take the train and a term based on the number of days I ride. Whoo, multiple variables! So I figure out that based on a fuel economy of about 45 mpg* and a 22-day work month, the sweet spot is about 6 train rides a month (one 12-ride) before the monthly pass is cheaper. Then I need to chart the effect of fuel prices on my savings. Because it turns out the 6 ride rule only works until gas hits $3.35. But I really can't see myself taking the train more than 5 times (one week) in a month. I like the schedular freedom riding gives me (no running to catch the train!) and so long as the free parking gods shine on me, I'm golden.
The net result was a nice 3D graph of cost vs. # of train rides vs. price of gas. No, you can't see it.
Also, of interest to no one but Tom, I'm no longer last on the BZ ladder (for now).
* It's usually right around 50, but I'm allowing for traffic, since my commute is basically all mAssPike.