Seized Eye

[Brion Toss]

Hi there,
You could easily get plenty of strength from seizings, assuming you make them correctly, and especially assuming that you apply them in the correct manner. That is, make the one furthest from the eye first, then apply a little load, so there's no slack between the two legs of the eye, and apply the second one, about 3" from the first. Apply a bit more load and put on the one at the throat. This arrangement should approach 100% efficiency.
On the other hand, the kind of efficiency you say you require (60 to 70%) can be achieved with a Buntline Hitch, or any number of other knots, and this might be preferable, especially if your horse causes the lead to fetch up at one end of the rope or the other; could chafe seizings. I'd consider hitching to a shackle pin, putting a second shackle on the tree webbing, then lashing to the between the shackles for tension. Make sure that the lead carabiner can't hang up on the shackles, or your horse could get stuck at one end.
Fair leads,
Brion Toss