I am thinking of investing in a refractometer to use in the brewing process. I've read that you can take samples during brew day to get your OG in the right place. I've read that you can use DME to get your specific gravity to where it needs to be. For example, if your OG is going to be too low you can add some DME to the wort to bring the OG up. How do you know how much DME to add or is the method to just add a little at a time while keeping testing on the refractometer?