The end of the firing pin will be nearly flush with the back of the slide. I just went through this with 75B 9mm slide. With a factory firing pin and firing pin spring and a roll pin retaining pin, with the barrel horizontal, and a well worn 17# hammer spring, a pencil will launch about 2-3 feet out of the gun. However, it was this way with a P-07 and P-09 (with light hammer springs) also, and all fire fine with normal primers.
It is easy to bugger up the firing pin with the retaining pin if it isn't pushed in flush when you drive the pin in. All you have to do, assuming the pin is not bent, is clean it up with a fine file.
Make sure you use a pencil with a flat eraser. A rounded one may not get hit hard enough because the pencil isn't centered in the bore, with the barrel horizontal. I never tried it with the barrel vertical, but that isn't a bad idea.
Joe