transfer function of the sensor
For something like this, you don't calculate it, you measure it.

You need to set up some mechanical system to allow you to vary the strain accurately, perhaps clamps and a ruler. Then you need some system to measure the resistance, perhaps a DMM. Plot resistance against strain. Over a small range, it can be approximated by a linear relation. Over a large range, I don't expect that will be possible.

While you are measuring, you will need to see the effect of various confounding factors that might make your transfer function less accurate. The effect probably has hysteresis (sensitivity to strain history), temperature sensitivity, and time (creep). Once you know the sensitivity to these, you'll have an idea of what accuracy you can expect from the system.