Glycol system help!

Hey guys. Green with glycol systems. Everything else I work on, so came here for help. Working at whiners tomorrow, Roma Ed pump on Friday, and have controls next, but I need help with glycol level. I used my refractometer and the temp of the solution was about 25 degrees. The lowest temp they run is 28 degrees, so minus 20-25 degrees, I should be looking for a 8-3 degree glycol temp? The tank is about 24" x 48". It's about 3/4 full. How much glycol do you think I'll need to bring with me? Am I right about this? Thanks for your help guys