The billing cycle is every 31 days, so it doesn't matter what day of the month you signup. If you start today, the month will be considered to have ended 31 days later.
Since it’s flex I don’t think it matters which testing center you pick, I just chose the one closest to me. But I’m pretty sure you don’t signup for a time until it’s closer to the date of the test
The option to practice flex wasn't available on 7sage before I took the July, but the PT I just took was a simulated Flex and I scored a personal best. I'm going to signup for October's Flex today, and just keep doing 2-3 PTs a week in the meantime.