Err, dude...that's bullshit
Reason: Flak bullets don't fly in a ballistic pattern towards a ground target, but more or less straight in the air.
In other words, if, with 1500m range, it was put at 1500m height, the
highest point it could hit would be 3000m. But the bullets would
not be able to magically fly twice as high, making the highest point 1500m height + 1500m range + 1500m magic gain == 4500m.
I know the logic behind this can be read from what you wrote, if you know what you're looking for, but wording has to be precise here: Flak shells should not get a range bonus from height. Yes, they should/will be able to hit a higher point in the air, simply because the gun itself is positioned higher - but they should not get an
additional bonus.