Not much help here, I tried those things a few years ago, thinned, not thinned, just couldn't get the thing to work.
Had a spare and tried it and that wouldn't work either.
Now I just use a gravity fed gun.
It works well because the drill bit puts a nice inside tapered end on the post.
Pull the dot snaps come in different post sizes so I like to have a couple to choose from and they roll over real nice with hand punches.
It depends on the strength you require.
A double fishermans will work but you won't be able to load it because it will pull back together until the two knots lock.
A prusik knot or blakes hitch will lock under load and is adjustable.
I usually tack the upper left corner down and strip the leather from the right side.
Make sure you have a new or very sharp blade.
Sometimes it's necessary to cut the leather into smaller manageable pieces.