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Then I would put a dot anywhere on the line preferably in open water.

Then through that dot pointing somewhere west of north I would draw a line through the dot so that the angle between my northwest line and my south east line was 130 degrees.

Then I would use a parallel rule to find out the magnetic heading of this second line.

The difference between the magnetic heading of the second line and 300 is your deviation.

There are real navigators on this forum but maybe this will get you started.

This is how I thought about it.

If you see the lookout tower and the Island in line then you have to be somewhere on a line that connects those two.

The only question is where on the line and which direction you are pointing.

We don't know nor care where on the line but we know we are pointing 130 degrees west of the line because that is what the Pelorus says.

Now we can tell what our magnetic heading should be.

If it doesn't match our compass the difference must be the deviation.

It looks like the second question is the same as the first just different numbers.

I'm just guessing here but does this make sense?