Leaving a job to start a business (run these 6 numbers first)

Most pages on this question ask about your character. Leaving a job to start a business comes down to 6 numbers, including the 1 your payslip never shows you.
Starting a business at 40 (10 advantages you already have)

The mean founding age in the US is 41.9. Here are 10 advantages you are already carrying at 40, each one you can check today, and the 2 things that get harder.
How to start a business while working full time (in a job you hate)

Most advice measures how much you dislike the job. These 10 items measure what is actually holding you there, plus the 4 things worth doing this month while you are still drawing a salary.