Calculation into variable format - syntax

[#1]

hi i have the following calculation which i wish to transform into a variable;

sum([Förs pris] *  "Förs-kurs" * [Ursprungligt antal] * (1 + OrderRad_Rabatt) * (1+ OrderHuvud_Rabatt))

i have tried creating the variable but am unable to reuse it in any expression, have i done something wrong?

Best,

Brad