rear spacer question

[Voa]

I have a puma on standard 15 inch wheels , lowered 30mm on gmax suspension and I want to add spacers to add some width to the rear of the car.Which size spacer should I go for 10mm or 12mm I am going to 16 inch wheels soon but not yet.The car has the later type beam being a 2001.

I have seen some people running 20mm at the back but I believe this will be too wide intention is to track the car at some point next year after prep'ing the car this year.