If I may chime in,
I made a spreadsheet table of the food that I generally eat. Given the GI, net carbs, fiber and sugar alcohols, the calculation of the FP is easy. And having my own table I can put in the precise values of the things I buy where I live which makes quite a difference in some cases. The values in my spreadsheet are all provided per 100g (which is what is provided on the food labels). Then I can easily multiply with the amount of what I eat.
So, yes, either I weigh what I eat (it can be fun, too, to see how far one’s guessing is off;) either before or after cooking, generally before, e.g. also with rice.
That’s because I assume, hopefully correctly, that the amount of carbs of rice doesn’t change much by cooking where we only add water which mostly is absorbed by the rice (contrary to cooking other veggies where water is not absorbed).
About sugar versus sugar alcohols (Norm, tch, tch, how could you miss this important question! 😉 Sugar alcohols (SA) are very different from sugar. SA is more like fiber. It’s practically not digested. Whereas “normal” sugar is half fructose half glucose and has a GI of over 50%. In short, we’d be careful with both but SA often adds even more to the GI. Of course, you can always just calculate it.