A few things I learned:
Use a dowel commensurate with the blade tang. I used a 1/2 inch for this Mazaki 210. Will use 3/8 for some pettys I will do next.
Take a silver sharpie and mark a straight line on top of the ferrule and the handle material before you glue up and after you get the blade true. Gives a good reference for glue up.
Make sure your blade and dowel slide in and out without a lot of friction. Check for the angle you want the blade canted relative to the handle. Take your time and get it perfect. Epoxy and beeswax will hold fine.
Use a Forstner bit to drill from the backside of the ferrule for the dowel. Drill this hole deep and leave about 3/8 inch to cut your slot into. Use a small drill bit to make a pilot hole from the other end to center the forstner bit.
Buy and use one of these to mark center of the ferrule
https://www.amazon.com/gp/product/B01D943QD6/ref=ppx_yo_dt_b_asin_title_o09_s02?ie=UTF8&psc=1
Use a drill press and go slow for any drilling.
Square your work with a saw not your belt sander. Taper on the belt sander and any small tapers on the ends of the handle do by hand on paper and a granite surface plate.
Use your calipers and measure and mark a lot.
Spend a lot of time sanding by hand on the granite slab and lower grits.
After all you do to get it right chances are the blade won't fit as you want when you finish.
Use your files and small rasps and a long drill bit if needed.
I used beeswax to fill up the slot, heated the blade, and will see how that works. I want it sealed but want to be able to remove the blade as well.
I don't have a ton of time so this took me a few days working when I could. Sanding is a *****.
I plan to eventually make some mammoth handles for a Full Moon Honyaki and a TF Denka.
Have 2 pettys up next and will post when complete.
Again, thanks to this forum and all the folks who have helped me over the years.