I want to make a diorama with a Russian tank rolling into Berlin. I want to have a PAK40 hidden in the rubble about to fire on the tank. Is there range where it's too close to shoot? In 1/35th terms, how may inches/cms is too close?

No, in a word. In Tunisia in 1943, a battery of 25Pdr guns firing solid shot engaged Tiger tanks down to 10 yards (30 ft), until they were overrun. So it's more down to how well camouflaged your gun is, or the tank could be turning a corner.

In my opinion there is no thing such as "too close" to try and kill enemy! You don't want to kill yourself in the process but in a situation like you described I think that you would want to fire when you were certain the enemy could be killed as long as you had adequate cover.