I guess you the front knob issue is probably lens specific. I have the Kirk replacement foot for the Nikon 200-400 and 300 f2.8 and don’t find it much of an issue personally. Regarding derating I mean the uniqball ubh45 was rated at 40kg load capacity. The flex line pro which as far as I can see is same physical size and design to the ubh45 but rated at 10kg (same as the smaller uniqball ubh35). With the bigger flex line extreme being rated at 20kg but larger and heavier than the uniqbslls at 1.1kg it makes me wonder if the balancing spring mechanism is the deciding factor on load capacity. There was a specific lens guide for the uniqball 35 and 45, just wondering if the same idea should be applied to the flex line pro and extreme.