Hi Andrew,
I am not claiming to be an expert here, so if I need to be corrected, please wave the BS flag. I am still learning.
OK I hear you, 100 JB and you want to insert a few (3' is perfect) of Mono - I like Fisherman Stealth Leader (no need for floro leaders, floro too stiff and wind knots would be a nightmare). Whip the serve only on the JB at the end where the mono enters the braid, DO NOT cross over to the mono. Use a nail knot with 30 JB solid (or equivalent) while UNDER TENSION! for full details, Pametfisher from 360Tuna has a great write up/pics on how to.
Length of leader; I find that if I keep the joint (where mono enters the JB) between my reel and stripper guide ( that will leave about <2 feet of mono+braid on the reel, works fine for me and i find wind knots are greatly minimized).
Try JB 80 lbs instead (80 has a slightly larger diameter than 100 - go figure right? its true). I find the 80 a lot easier to work with esp the mono insertion.
You can then prepare as many as you like leaders with about 4 feet of JB from where the mono ends in the braid and if needed do a spectra hollow to hollow joint, here's the link
http://www.dahoproducts.com/How_To_Use.html#2.2._Splicing_Spectra_Lines . Of course using a splicing needle on a rocking boat may not be ideal, but it can be done, see the link and you know how easy it is.
This also works if you have solid spectra on the reel, then you'll just insert the solid into the hollow 1' from the end and use the free tag end to do a 6 turn rissio to secure the hollow on the solid. I primarily use this set-up.
This set up cast like a dream. However, if you do a lot of casting or land a big fish, change the leader out at the end of each day.
Hope this helps
Good fishing!