Calculate concentration gradients in FPLC Our FPLC works with two solutions:

A: 20 mM Tris.HCl, 0.2 M NaCl, pH 7.8
B: 20 mM Tris.HCl, 1.6 M NaCl, pH 7.8

I'm interested in sample volume from 102 mL to 117 mL. The FPLC will have a linear concentration gradient: At 24 mL it will be 75% A and at 144 mL it will be 5% A (the rest is filled up with B).
For visualization, I made a graph (y = -7/1200*x + 0.89):

Now, I have a sample with the whole output between the two indicated points (102 mL and 117 mL, so I have 15 mL in my tube) and would like to calculate the NaCl concentration.
Percentage of both A and B would change 70% from 24 to 144mL, with A from 75% to 5% and B from 25% up to 95%. The change rate would be 70% during the 120mL period or $0.70/120$ per mL. At any given point $x$ mL, between 24 and 144mL, concentration of $\ce{NaCl}$ in $\ce{mol/L}$ is:  $$\ce{[NaCl]} = (0.75 - 0.7\times (x-24)/120)\times0.2 + (0.25 + 0.7\times (x-24)/120)\times1.6$$ 

For a 15mL solution between 102 and 117mL, the concentration should be the average of the two points when x = 102, and 117mL.