I've been making these damn things for years.
But when I started I had to teach myself with no pc or outside advice.
All I had was a few look-sees at what I came across on a charter in Mexico.
I experimented and tested until my wife thought I'd break the garage floor with the crump of my test rig going off!
Now I can make a 4inch splice, with only 2x1/4inch glued sections, that will take a full 56lb weight off the floor AND jerk it up and down without separating. (and YES that is a four inch splice)!
My experiments made me realise that the answer didn't lie in the LENGTH of the splice but in HOW TIGHT the splice gripped the mono.
So here is my 6cents worth - make of it what you will.
*Spectra will not grip the leader material as tight as nylon.
*For the loop, whippings and leader use only nylon.
*Nylon can be 'shrunk' using easily available rod building materials.
*The leader inside the splice can be weakened by 50% (because the strength is a combination of leader AND sleeve material).
*A new wind-on leader for each good fish demands a quick production time.
So, to make up,
Use a scalpel to make slices into the mono to MAKE it grip inside the nylon sleeve.
Use high quality, hollow core, nylon (available from English Braids UK).
Make 2 or 3, 1/4inch 'out and back in' passes with the leader material as it goes up the sleeve.
Whip only these 'in/out areas with fine rod guide whipping thread.
Shrink the whipped 1/4inch areas onto the leader, using thinned guide wrapping epoxy polymer- 2 coats.
Whip the splice end as normal.
I make my loops 'Tanaka style' which eliminates the doubled sleeve material and makes a smoother and thinner loop joint.