If you ever get AOB higher (to the left of) 90 you are doing something wrong. Of course, the real AOB can be higher than 90 but the result from your calculation will not. When your result is 70 it could also be a real AOB of 110, 40 could also be 140 and it's up to you to make that call. When dragging the mast height to length of ship it might be easier to think about it as dragging the measured distance to the length, since that number will actually follow your mouse when you move it and it doesn't matter if you slip with the mouse, you can always check that the measured range really aligns with the length of the target when you are done.
When you measure speed you do not need to be at almost exactly 90 AOB as someone suggested, anything between 45 and 135 will give very accurate results. It can be higher or lower than that too as long as you can clearly make out where the "tip" of the bow and stern is. Measuring from somewhere bow-left and to stern-right will underestimate the speed, and this could be by a large margin.
The same goes for you own speed. One knot for you could mean that you under or overestimate the speed with much more than that, twice you speed at an AOB of 30. The best bet is to always measure at or very close to a bearing of 000 or 180. Then your speed wont matter, and it's actually better to cruise at 2-3 knots than beeing at a complete standstill, you keep your course and don't drift in the wind.
Your sonarman can also be of some help. Slow is always slower than 8 knots, medium is slower than 12 (for warships this is 18)and fast is slower than 35 while very fast is 35+.
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