Sankt Petersburg (Leningrad then) would be an obvious choice...

From your data (40km detection against IL-28, half-rcs of IL-28) and the basic radar equation, the detection range of U-2 would be

R = 40 *(1/2)^(1/4) = 33.6... km.

So, the answer is 33-34km. Probably, still within the capabilities of the first-gen Berkut system, but the engagement window is very small and it is easy to slip through.

Btw, from the vid that you have uploaded, one sees nicely that the tracking operator radar scope is basically an oscilloscope (24 cadres per sec is enough to see scanning beam) biggrin