I just keep the same half way point diameter for male spigot , extended by 1" & parallel.... Female is purely .040" per strip over the halfway diameter....
I think some tapers show a flat spot more than others, due to the length of the swelled portion leading up to the female ferrule, so in respect of that , i fins these rods can work in a few different ways which i soon came to realise... For certain aspects and circumstance on stream,one can optimise the tip portion throughout their cast, for different tight casts etc , where longer casts will start to optimise the flex below the spigot & fighting a heavy fish will soon harmonise the taper throughout... Its an oddity , but when you are out fishing these things you come to realise , like any rod, you get used to it... If you only ever had one rod, you make it work & you get used to it.. For small streams , rivers and mountain lakes, these are the rods i fish , i fish nothing else as I enjoy fishing this ferrule type very much..
I did try the reduced spigot method, wasnt 100% happy with what i did... Maybe i will revisit that someday

I am a fisherman, and for my fishing i make rods that cast a fly and catch fish.. If i dont have to buy or make silver ferrules it is a bonus

that is why i love making a bamboo ferrule sleeve, the material is already available....
I havent yet made a seperate form to make Bjarnes type ferrule but i can see me doing so in the future... He was my inspiration many years ago now, & still is...
I think the bamboo ferrule like this does make the rod feel more parabolic , even without the newer reduced portion compund? method... It surprising how many tapers work without modification
How are you going about your ferrules Taniwha. Are you making them the same way as i do? How do you find them to be?
Cheers