Thanks TomGeorge and DVDDouge for the feedback.
Given the cheap cost of electronic components, it would be be best for me to perform tests on the scooter itself to see how things go.
As for my Math, it seems I forgot to account for the 100R resistors affect on the input of the Voltage regulator, though I don't know how to do that at the moment.
The mentioned Scootputer project directly connects the 12v battery to the Arduino Board but I wont be using an Arduino Board, just the chip.