I've been through several threads on Catcher Pitch Call ratings but the one question I still have is this: What is the LOWEST Pitch Calling rating I can get away with at the ML level? How low can I go before it hurts my pitching performances and does PC really matter as far as a pitcher's game performance?

I'd say somewhere around 40. I've used high-40s catchers in a pitcher's park with relatively little observed detriment. By that I mean, my team already had the best performing pitching staff so how much more would I want?

Quality of pitchers, park factor, quality of defense, and strength of opposition all factor into it. So for me the decision, playing in the National League, has always been whether I need my catcher to be an actual offensive contributor or not. The catcher I used with PC of 47 was top three on my team in OPS.