The thing to keep in mind here is that the guns on the original were mounted BELOW the level of the cowling and that the cowling was mounted AROUND them. The barrel therefore should be just barely above the surface of the cowl. At your scale something like 1/8" I would imagine. To just stick it on full height looks just plain ridiculous. You might as well just duct tape a 50 calibre to the wings of a P-51. But this has somehow become an enduring modeling misconception and 99% of the Eindeckers out there (including the one on the SR website) look toy-like because of it. I recommend that you have a HARD look at photos of ANY German WWI aircrraft with Spandaus.

For modelers this means one of two things. One, you can actually cut holes in your top decking and mount the guns below the surface as per original. Or two, you'll need to cut off about 1/3 of the bottom of the rear of the gun. If you look carefully at the angled forward attachment point on the Spandau you'll see then the support piece starts to angle back. The surface should just where this angle starts.

Or you could just stick it on the top like everyone else and give up all pretentions that you're trying to be scale. In fact you could mount TWO guns like that 'cause it would look really cool.

Sorry. One of my pet peeves.