The HHG_cutoff.xlsx is the spread sheet that calculates the KE of the returning electron given v_o/v_Q. See pg 6 of the Chap1.pdf. This calculation does not include the acceleration from the Coulomb potential so it is approximate. The highest KE is slightly less than 3.17 U_p, the known value including Coulomb potential.

Given v_o, you need to first find the time where x(t) = 0. This is where f(phi) = 1 - cos(phi) + a*phi = 0 with a = v_o/v_Q and phi = omega*t. The spread sheet uses Newton's method to solve for the zero of f(phi). From plots, I know that phi~4.5 for a~-0.25. Vary the value in cell D2 to get the factor multiplying U_p in the HHG cutoff (cell E17). Once you have phi, the KE is
KE = (1/2) m v_Q^2 (a + sin(phi))^2 = 2.0 (a + sin(phi))^2 U_p
The best value for a is between -0.25 and -0.35.

The mathematica notebook HHG.nb will let you see plots of the scaled position, velocity, and KE vs time. It also will find the times (in units of phi = w t) where the position is 0 and the KE for that value.

The excel spread sheet finds one zero for a ~ -0.3 using Newton's method. It also uses the zero to calculate the KE at that time.
Newton's method:
If x is near a zero of f(x), then
x_0 = x - f(x)/f'(x)
is a closer value for the zero. You can see this by Taylor expanding at x_0
f(x_0) = f(x) + (x_0 - x) f'(x) + ... = 0