Ehhhh.... I have two MAP BA chassis and the bubble/bean/whatever is about useless imho. Neither of mine were level when I checked them with a much larger level across the rail on barreled actions installed in the chassis (kinda sorta fixed by popping the little thing out and shimming it with tiny plastic tape). You can't see it with the cheek welded in firing position (at least I can't). And finally, it's so short that I wouldn't trust it anyway - like trying to draw a 3-foot straight line with a 2-inch ruler.
It's a nice gimmick, though.