Wtih meatballs, meatloaves, sausage, etc., you "try" it, by frying a very small amount in a pan and adjusting the seasoning that way. You'll develop a "feel" for how much looks and feels right without testing on everything but sausage.
With things like grape leaves, just grab a very little salt and sprinkle it by hand into the mixture, then use as much in the cooking liquid as you'd use to cook rice -- less that first pinch. In the years I've been following your cooking adventures, koukouvagia, I know you don't nead more than that.
With baked goods it's a good idea to develop and follow some rules, but this thread isn't the place.
No criticism intended, as Leenie seems like a fine cook. But suggestion makes sense only for people cooking for others with no tolerance for salt. It's poor technique generally and leads to flat tasting food which can't be entirely repaired at table. Ideally we like to season in layers -- and by "season" I definitely mean salt and pepper more than anything else, and by "layers" I mean we start at or near the beginning and taste and adjust throughout the cooking process. But nearly always, we add at the beginning.
It's been said that the goal for the homecook is to salt to the level of the person at table who uses the least. However, I think that applies only if there's someone very sensitive to salt. While it's hard to define, there is a sort of narrow, "ideal" seasoning range and an ideal balance of salt and pepper.