I just put a left hand PFS stock with the ultimate comb on my MX8 (low rib).
I need quite a bit of cast on so it is adjusted almost all the way it will go.
When i mount the gun I can feel the heads of the screws on my cheek.
You need to unscrew the upper bar, slide off the block, flip it 180 degrees (not upside down), slide it back on bar, tighten averything back up and you should be good to go. That is how I have mine mounted and it works really well.