I geek out over scientific equipment for some reason and managed to find a really really nice Mettler Toledo balance from the early 2000s(?) (PR203) instead of doing the spoon thing. Very repeatable results from it and from what I understand thatís a nice thing if you want to really get dialed in. Not necessary though
If that's something your into than more power to you, but it's called 'Estimative' for a reason. It's not an exact thing. In fact from day to day the concentrations will change anyway based on uptake, plant mass, growth, water changes, etc. Even the gallons your measuring for are off based on displacement, etc.