Definitely a useful guide, but i do recall Howard Crostan matches fly size to required insect size (or use small if he's fishing through already fished water) and bead size to water conditions. In theory he could have a 4mm bead on a 16, for example.
I frequently use 3.5mm on a 18,
assuming gape is fine., which with a jig hool and slotted bead it is. I dont think I miss anymore fish than normal. When overbeading i do go black rather than bright.