In reply to Ste1:
Probably more like 1.3-1.5 for both locations, at a very rough guess. Looking at the table at the bottom of this piece, London is currently 1.4, and both the NYM and ND are a similar longitude. The difference between mag and grid is steadily closing in this part of the country.
https://www.ukhillwalking.com/articles/skills/series/skills/magnetic_north_...
Personally I have never bothered adjusting if it's only a degree or two of difference. It's unlikely you'll ever be able to walk that accurately on a bearing anyway. Besides which, if you keep your legs relatively short then the cumulative error will be negligible. Also, navigation for most walkers is a rough and ready thing - you're not often aiming for a needle in a haystack, the targets are usually large enough to give you a bit of margin for inaccuracy either side. So I'd say it's something to be aware of, but not to lose sleep over!
Post edited at 10:49