I found that if you filled to the bottom edge of the front plug, you would soon find oil blown out past the seal collecting in various depressions on the front of the S/C. When I had the install done by Alpine in their shop, they filled by volume and that resulted in the level being 1/4" below the front plug. When I did service later, I determined this by lowering a small tube on a syringe 1/4" into the gear case and I sucked back oil to that level.
Roy