With an IG do not drain below the returns. That is pretty low. No need to. How far you drain depends on your anticipated water table level over the Winter and other factors. One factor being can you periodically drain during the Winter and do you get a large ice block in the pool. Draining with an ice block can be tricky. With a liner pool the relative water levels is even more important. You can easily float a liner. I really am working on a sticky for Winterizing IG pools similar to what Watermom posted for AG pools.
Al