Hello, foam on a tire can be about 50-60g per set, depending how big you start.
This is how i am doing it:
i.e. you start with 62 in the rear for qualification,
you know your tire wear in a 5min run is 2.5mm, so you come in with
59,5mm in the rear.
I am using tires with a diameter of 56mm for minimum weight calculation in qualification. For finals i am allways useing blank rims
Dont forget: there a massive weight differences between rims, carbon rims are much lighter than the plastic ones from a controlled tire, so make sure you are doing it with the same rim you are useing later in the race.
Also make sure you control weight with a clean shell and car as there can also be about 8g and a fully empty tank.
Greetings