I've found many equations during my research but am unable to find one that relates to the LM358.

The most common equation I have found is: T=2RC ln[(1+λ)/(1-λ)] Where λ=R1/R1+R2

Of course this does not take into account R3 of the LM358 circuit. Can I leave R3 out of the equation or do I change the equation to this: λ=R1/R1+R2+R3 ?

Any help would be much appreciated.