As you will see below, there is insufficient information to be able to calculate precisely how much dough is used to make an 18" pizza or a 16" pizza. However, I think we can get quite close.
If 900 grams of dough is used to make a 24" pizza, the corresponding thickness factor is [(900/28.35)/(3.14159 x 12 x 12)] = 0.070174196.
If 12 ounces of dough, or 340.2 grams, is used to make a 14" pizza, the corresponding thickness factor is 12/(3.14159 x 7 x 7) = 0.077953507.
When Luc suggested dividing the 900g/31.746 oz dough ball into four pieces, I believe that she was actually giving you the amount of dough to use for a 12" pizza. Assuming that such was the intention, then the corresponding thickness factor is [(900/28.35//4)/(3.14159 x 6 x 6)] = 0.070174196. That is the same value as for the 24" size.
As you can see, the three thickness factors given above are quite close in value. To determine how much dough is used to make either a 16" pizza or an 18" pizza, you would have to pick one of the three thickness factors, or possibly average the three values and use that number. Or, you could ask Luc sometime what dough ball weights she uses for those two sizes. For now, if we were to use the average of the three thickness factors just to get some rough numbers, the amount of dough that you would need to make a 16" pizza would be 3.14159 x 8 x 8 x 0.072767299 (the average) = 14.631 oz, or 414.78 grams.
In like manner, the amount of dough that you would need to make an 18" pizza would be 3.14159 x 9 x 9 x 0.072767299 = 18.52 ounces, or 524.96 grams.
I used all of the decimal places from my calculations in case I have to revisit the math.
The best data for the 16" and 18" sizes would be Luc. If you can get that, then we can fine tune the values even further. But, whatever the outcome, the thickness factors are in line with the values that are typically used to make an "elite" New York style pizza.