no, you see, that's the thing about speed.
as long as your periscope is pointing exactly ahead of you, or exactly behind you (0 or 180 degree bearing)...
the ship will travel exactly its length, not 'thousands of meters'
i understand your confusion, but you have to realise that if the ship moves at a 45 degree AOB, the 'size' of the hull through your periscope is smaller than if it were moving at 90 degrees AOB. it will always need the exact same number of seconds to move that size of hull through your 0 bearing line
take a pen and and move it around on your table, move it at various angles, move it at various ranges, move it any way you like, as long as you move it in a straight line. hold a finger at the tip of the pen and keep looking at your finger as you move the pen, if you keep looking at one spot as the pen moves across that spot, it will always travel the same length: which is the length of the pen, which you can time to measure its speed
reading back, it's probably not as clear as it could be, but I hope that helped
oh, the subtract from 180 thing is when the target moves away from you. 70 degree aob means it's coming at you, 110 aob means it's moving away from you, the 'size' of the hull is the same in both cases and the periscope tool will only give you '70' so you have to make that '110' yourself, by subtracting 70 from 180