Basically the value of a commodity is calculate by c + v + s. c being constant capital (the value of the means of production that has been degraded to create the commodity, hard to calculate without good info), v is the amount of paid labor (variable capital), and s being surplus value is the unpaid labor.
To calculate the value of your labor all you need to know is the constant capital used to produce whatever commodity you produce and the value of said commodity. By including your wage as v you can calculate the surplus value and then compare them to see what share of the value you produce you actually recieve.
It gets harder when you aren't directly producing commodities which my friend was.