But as you workout you’ll become fitter, or sometimes you get sick and need a couple weeks off. Anyway, you need to do an FTP test at least once a month to recalibrate the numbers. You can’t use a number that is over a year old. Not sure what you mean by ‘great accuracy’, because the accuracy of any given FTP test method will have some issues that influence the accuracy, but you don’t know at all if you don’t test.