A gauge is just an estimating tool to help you pick feathers that are
'right' for your hook. And if it looks too small or too big on the final
product, use a bigger or smaller feather for the next one.

I mentioned this a couple of weeks ago, but since it may be a highly
useful piece of information, I'll risk repeating it: For the past
year or so, Goerge Harey has been preaching the merits of dry flies
tied wth hackle *two* sizes too small.

I must confess I haven't tried that, but I suspect such flies work for
the much the same reasons comparaduns do.

vince