OP is like a CEO, quarterly bottomline is the only thing that matters. There is nothing wrong with this approach, but not accounting for all the variables give a really false sense of savings (or lack thereof). As many pointed out, OP's monthly increase is not solely for the new Tesla but probably includes other increase in usages such as air conditioning. Looking back at the last few months, I can see I have turned AC on for the house at least 10+ times. That easily translates to 300-400 kWh extra usage.
If I was to believe TeslaMate (self hosted TeslaFi), it took me 104kWh of electricity for the last 302 miles of driving. This assumes all inefficiency, vampire drains, air conditioning, sentry mode, whatever. I am constantly amazed at how often the car is "awake" with some sort of pump running. All those takes energy but contributes nothing to the actual driving. If you use the figure above, I am only getting about 3 miles to each kWh pumped in. This is roughly $0.03 to $0.04 per mile for SDG&E EV-TOU5 rate (not including the monthly $16 charge for the privilege of the $0.09 super off-peak rate). At the end of the day, this is still cheaper than the per mile cost of a Toyota Prius (which roughly doubles/triples to $0.09 / mile @ $4.30/gallon gas for 50MPG).
If OP has to charge the car with higher cost electricity, there may not be any savings. Once again, depending on the SDG&E tariff choice, it can cost as high as $0.56 / kWh for TOU during summer and even without TOU, 900kWh usage may put the cost in the $0.40 range. This is on-par with Super Charger's rate at $0.41 in most San Diego. There is NO SAVINGS if OP keeps pumping in expensive electricity. This is the joy of living in SoCal.
As many has already pointed out, OP needs to dig in just a little bit into the monthly SDG&E statement and see which tariff they have and potentially figure a way to get cheaper electricity consistently. TOU only make sense if you can avoid usage at 4pm to 9pm ($0.56/kWh). Imaging running your AC during those time!