How do you calculate how much DME to add in order to hit your OG if you are using a refractometer?
Today I took refractometer readings at 0 min (6 plato), 15 min (7 plato), 30 (8 plato), 40 min (between 8 and 9 plato). I was aiming at a FG of 11 plato, so I added a .5 lb DME in order to try to hit my OG as sort of a guess using Beer Tools, but was unsure at the time if it was right. Using a hydrometer after I adjusted for the temperature the OG was 1.045 (about 11 plate).
Is there any formula for how much DME to add at a certain time when you realize you aren't going to hit your OG? Examples would be fantastic!