Couple of other useful tips regarding diversions: (1) Treat your current (known) position with (hopefully) a good landmmark as your 'local area' and don't set off from it until planned & ready. Once you set off, if it looks like you've made a gross error calculation then do a quick 180 and return to your known point to figure out your error.
Other tip (2) is to always note if you are north/south or east/west of good line features such as motorways/canals/railways etc. That way you will always know what 'grid sector' you are in.