The first joint is generally used for a revolver. A semi-auto with a short trigger like a 1911 clone you should use the pad of the finger closer to the tip. This area of the finger has the most tactile response.
With a pivoting trigger like your H&K, depending upon the trigger pull weight, it may require a little more finger closer to the joint. Just remember to pull straight back without any added pressure to any side of the trigger which would pull your shots left or right.