Determining angle on bow - the realistic way
 

Just wondering, what's the most realistic way to determine angle on bow? Is best to make an educated guess and fire a spread? And if so, what's the best spread angle?

I'm no expert here but from what i've read here and there both US and German had AoB calculator rulers. However, at least americans (not sure about germans, although i recall reading that some did the same) were used to eyeball it (faster and rely mostly on your skill rather than knowing ship length and height).
If my memories don't trick me japanese had a device in the periscope to help them to ESTIMATE it.

Consider that calculating a fire solution was not something that you set once and you are ready to go, it requires constant updates so eyeball it was the fastest way to gather it.

BTW, i wouldn't call it guess because guess imply random and big error...XD. I would call it estimation which imply a small error (within 5 degree as worst average) .
It takes a lot of practice and you should know where to look for to estimate it (wave, mast, bridge visibility, bow, etc...). A fast aob calculator helps a lot in the learning process (the fastest method i know is the nomogram but it implies you know both length and height).