How to do a theoretical calculation of brix?

Hi, I understand that brix could represent total soluble solids content. Is it possible to do a theoretical calculation of brix? Besudes sugars, are preserbatives, liquid colouring, vitamins considered in the calculation as well? Can i take it that they contribute 100% (i.e. 1g of vitamin c contributes 1brix)? Thank you.

"Maybe", is the definitive answer

Are you specifically considering refractometric Brix, or are you looking at "actual" total soluble solids?

The former, being measured based on the refractive index of the liquid, will interpret things other than sugars in rather differing ways - for example, your vitamin powder may well be 100% solids in common-sense terms, but e.g. a 50%w/w solution of it will not register as being 50 °Brix unless it happens to have exactly the same refrative index as a 50%w/w sucrose solution.

Often these types of ingredient are ignored from theoretical Brix calculations unless they are used in unsually large quantities (for example, a highly concentrated compound base), as e.g. 200mg/L of potassium sorbate in an RTD is going to have a neglible impact even if it is 100 °Brix.

Nonetheless it's always worth doing a lab blend and measuring it to check

Acids are potentially worth considering though, as these can have a supressing effect on the refractometric Brix value, and as it's relatively well characterised you can fairly easily adjust for it - for this reason you'll sometimes see both a refractometric Brix value and a so-called "corrected" Brix value for some types of product. The latter is obtained by measuring the titratable acidity as anhydrous citric acid, and then looking up the corresponding correction value in a table.