I tried writing a letter to myself 10 years in the future, but it was too long a period to feel relevant (my predictions were hilariously off). So I tried five years and discovered my prediction time horizon was only 6 months long - any longer than that and I couldn’t even predict what I should be predicting. The exercise you suggest Grandma is a great one (foregone conclusion of course), not requiring any real prediction ability. Just pretend your ideal reader is out there right now, reading your words for the first time. Feels great doesn’t it?