grid to mag add, mag to grid get rid
Quite often it was hard to work out the mag variation depending how old the map was but depending on the length of the leg to march on a bearing and by aiming off a bit it often made no difference
Navigation is more an art than a science. I delivered a boat to St Peter Port from Littlehampton with the new owner, it was his first time that far from land and he had a brand new Windy Mirage, lovely boat. All the electronic whistles and bells and computery GPS gadgets you can think of. I got my chart out, laid a a course and used the the compass.... he asked if I was going to make allowance for the tidal vector. As the variation and tide were working against each other, no, I couldn't be bothered and we would be fine. He was having kittens. We heaved to about half way across the channel for a pint and a pastie that I had brought with me.... we drifted for half an hour and he was worried that our course would be all wrong. We set off again on the same bearing... the casquets lighthouse, my waypoint, came into sight bang on the nose! People try to make it too complicated. Christ Alive, Drake had a magnetised needle stuck in a cork floating in an upturned coconut shell for a compass!