How to define a threshold defined between two moving position
 

Hello everyone, I am new in the forum. I am simulating the diffusion in an interface between two gas with a 2D simulation .

In order to extrapolate the data, I defined a threshold with the mass fraction of one of the gas between 5% and 95%. Now, I would like to define a new threshold.

The new threshold has a lower limit, the minimum of the previous threshold minus 10% of the amplitude of the previous threshold, while the upper limit is the maximum of the previous threshold + 10% of the amplitude of the previous threshold.

I am a bit confused about how to do that. For now I defined two functions:
function 1: min - 0.1*amplitude
function 2: max + 0.1*amplitude

Now, I would like to define a threshold, where the position (Centroid (x)) is in between the two functions.

Has anyone any idea of a possible solution?

Thank you very much for any ideas!