Plot a formula

Hi,

I am trying to do simulation of resistivity changing versus deflection of a membrane. the membrane is subject to an
input pressure from range 0 to 100[kPa]. I simulate the model parametric and during the plot of any function same as resistivity, I need the value of that function in the first point of range to some calculation. How I can do this, I mean how I can do this by writting a furmolla
for example
Pressure range: (0, 10, 100) "(first point, step, end point)"
Function: V "Voltage"
I want to plot this formula:
(V-V(First point))/V(first point)

Any help will be greatly appreciated.