mathematically, an inverted parabola that passes thru the origin can be written in the form
y = kx(R-x)
where k is a constant and R is the 2nd x-intercept (the "Range" in this case)
for point (b,a) ...
a = kb(R-b)
for point (a,b) ...
b = ka(R-a)
now use the two equations to solve for R in terms of a and b.