Thank you Borek for your effort, but unfortunately that is not accurate enough. I am currently using polynomial equation to get conductivity of higher concentrated KCL solutions, but I am looking for an exact answer.

This is pretty close between 5% and 25%:

-6,8672*POWER("mass-percent",2) + 83,01*"mass-percent" + 0,1971

This is 3rd polynomial, which is a bit closer

= -0,3741*POWER("mass-percent",3) - 3,5001*POWER("mass-percent",2) + 74,181*"mass-percent" + 6,4824

Errors are 0.56% and -0.09% respectively based on math. But I am looking for an answer based on fact.

"13.7/15*208" is too far off.

A