Hi everyone,
I just did a ramp test, but I feel like I could have performed; when can I ideally retake it and how should I prepare to ensure I get the best results?
Take a couple of easy days, eat plenty of carbs, slam a double espresso and go for it. 
Even better, just start a free ride on tempus fugit, ride hard for 40 minutes and use the average power for your effort to manually set your FTP.