I am trying to do isobologram in R.

I have two drugs with difference maximum effect so I need to use this equation.

EA, EB -> maximum effect of drug A, B.

CA, CB -> IC50 of drug A,B

a, b -> fraction of drug A,B in combination

b = CB - CB/((EB/EA)*(1+(CA/a)))-1)

Which give curved isobologram at IC50.

But I also want to do it at IC25, IC75, IC90, etc. How to adjust the equation?

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3379564/