It can be very useful to quickly calculate the reciprocal of a course, but if you are like me, adding or subtracting 180 degrees from the course I’m on is beyond my normal mental capabilities. In other words, I can do it with a calculator or on paper but not in my head. Unfortunately, those times I typically need to do it quickly I have neither.
Here is an easy way to make the calculation in your head. Take the course you are on and either add or subtract 200, then do the opposite math operation and subtract or add 20 back. Works every time and it’s quick.
Here’s an example: You are going from the exit to the Potrero Reach across to Paradise Cove on a heading of 247 magnetic when the fog rolls in. 247 is greater than 200, so I’m going to subtract 200, leaving 047, then add 20 back. I end up with a course to steer of 067 to get back to the entrance of the channel home.
Hopefully, you remembered to calculate what the currents would be doing before you left!