I would never be able to eyeball the correct quantity and I’m not sure how you would convert weight to teaspoons.
You should be able to calibrate yourself if you saw the proper amounts laid out a few times. However, there would be significant variation if you were trying to put a little bit in your hand. Using fractional measuring spoons isn't a terrible way to go, but its far from the best and its way better than winging it with a dash in your hand.
In my opinion, there isn't much detriment to adding too little of a mineral to your water. But there can be big problems if you add too much. A teaspoon of this or that could be way too much for your water or your batch if you haven't done the calculations to assess what the proper dose is.
But in the end, you will still make beer and it will probably be drinkable. Don't get too tied up in dialing in ion concentrations to within a few digits. Being 5 or 10 ppm off is really no big deal for most ions and you can up that to 40 or 50 ppm when it comes to calcium. RDWHAHB, but try to avoid overdosing.