PRESS PHOTO BY NATE JASTRZEMSKI Tom and Ellen Flynn, in the spacious basement studio of their new amateur-focused creativity warehouse called The Art Establishment, say one of their eight resident instructors is teaching an ancient Greek painting method that mathematically determines the use of space on the canvas. A labor of love, the 945 Broadway Establishment took two-and-a-half years of painstaking refurbishment to finally be ready for use. The Flynns, enthusiastic amateurs themselves who work with oil