I see no need at all to apologize for using T-Max developer. T-Max developer and T-100 film make for an extremely flexible combination with nearly grainless negatives in most cases. My "normal" time for T-100 roll film is 10 minutes in 1:7 mixture. If I had a flat lighting situation such as you describe, I'd probably increase the time to 11 or 12 minutes. Good luck.