This involves determining the height of an isosceles triangle. The "height' is the distance from the base tab line to the joint where the two arms of the Aventa come together at the pivot. Comparing the new and old heights distances will tell you how much closer it will be.
For an exact answer, calculate as follows:
The length of the tow bar side arm is T. Square this number (T*T)
the Base tab length (B) and square that number (B/2 * B/2)
The distance from center of the base tab to the junction of the arms will be the square root of T-squared divided by B/2-squared
This article explains in more detail:
I don't know the length of the Aventa arms (when extended0, but the SWAG of around 2" should be fairly close. It's going to be inches rather than feet difference.