To be sure before purchasing spacers, if you can source one of the alloys with a tyre on it, put it on the back and check, then put it on the front and check, and as I posted on DaveJau1's recent thread 'Wheel Spacers:
Offer a piece of wood vertical from edge of wheel arch to ground (back and front checks), then measure horizontal gap between middle of tyre wall and inner edge of wood. This will give approximate size of spacer required. I guess it will 20 mm as you quoted above.