Jeremy,
In order to arrive at 500 ml of developer, the simplest way would be to have 500 ml of water and 5 ml of each A and B. Granted this will give you 510 ML but that is probably near enough for what you want.

If you want to get this nearer to the 500 ml total, then you would need to have 490 ml of water and 4.9 ml of each A and B. That would give you 499.8 ml if you have something that will measure that precisely...Good luck.