R-2R Ladder Resistors Mismatch
First thing to do is writing a function of the setup, like 

$$F(R_1, R_2, R_3,...R_N)=....$$

Then you have to calculate he partial derivatives for each element and multiply with resistance tolerance value:

$$\dfrac{\partial F}{\partial R_n}\cdot R_{nTolerance}  $$  

Then a reliable error limit would be:

$$ E =  \bigg[ \lvert \dfrac{\partial F}{\partial R_1}\cdot R_{1Tolerance} \rvert + \lvert \dfrac{\partial F}{\partial R_2}\cdot R_{2Tolerance} \rvert+\lvert \dfrac{\partial F}{\partial R_3}\cdot R_{3Tolerance} \rvert +....+ \lvert \dfrac{\partial F}{\partial R_N}\cdot R_{NTolerance} \rvert\bigg] $$