I'm certainly not a gun builder. But I know there is a rule of thumb regarding the height of sights. I believe the front sight should be about 1/16" higher than the rear sight. Something like that. Is there a rule of thumb for the approximate width of a front blade sight based on the width of the notch in the rear sight? I have filed a front sight to make it more narrow using trial and error. I have been having a little trouble with the daylight on either side of front sight, and I would rather not change sights.
I don't believe there's any rule of thumb other than one files their sights to the height(s) they need to be when zeroing in (your choice of distance)... All of my "Rifle's" top front blade sight's are just about 1/16th inch lower than the top of my Rear Sight's "V" notch / measured from off the top barrel flat...
However, I shoot barrel lengths from 28" through 42" and I doubt you'll find any two rifle's (sight heights) are the same - most likely because of barrel length and the Rifle's comb drop which in my case(s) are anywhere from 2-1/2" through 3-1/4" ... One needs to take into consideration as well, the Powder Charge they zeroed with and the ball weight they are using (example; .440 vs .445). All these factors come into play - as does shooting your zeroing in... If zeroing in - in a "humid" climate,,, your ball will shoot high in a dry climate" - also the "sea level" at where you shoot will come into play... There's a whole lot of factors that come into play.
Of course there's also "dead center & 6 o'clock hold" that can play into the factors of your preferred aiming stance / as well as the elevation you're shoot from... I'm a fixed sight's shooter, so I have to rely on my best judgement of my hold - dependent on the distance of the shot after reading my shooting condition... I can't explain it very well, but I can do it from 40+ years of experience...
Jim Beckworth's Hawken Rifle... Notice how low his Front Sight is as compared to the height of his Rear Sight.