Actually, there is no "Fudge Factor". A Transformer is one of the things that you can count on for 100% ampacity, for 100% duty cycle.
If you have a 600va load, you will never get there with a 500va transformer.
What is your application? 500va is only 1/2 amp at 120vac.
If you are working on ONE machine, buy a transformer with spare capacity. If you are engineering a purchase for 10 or more machines, then you must understand your load, and size the transformer for adequate capacity, and economics in design.