All that 5/8 th stuff is only because someone felt obliged to put a number on the width/thickness to stop people claiming flatbows were ELBs.
If you check your American longbows you'll probably find that is with the 5/8 rule anyway!
If you just make an ELB using your eye, you'll find it's well within the 5/8... you won't need to try and make it to the 5/8.
Generally they come out nearer 6/8 (or 3/4 as we sometimes call it
)
Just for info here are some dimensions from a 70# ELB
Yew ELB 71ntn 60#@28” 70#@31”
Left column, distance from nock (inches), then width and thickness in mm
4 16.9 14.5
8 19.3 16.9
12 21.4 17.4
16 23 19.2
20 24.7 20.6
24 26.1 21.4
28 27.2 22.2
32 27.7 23.3
34 28.2 26.2
If you check some of those figures you'll see it's nowhere near 5/8 (0.62)
E.G Mid limb 16" from nock 19.2 / 23 boils down to 6.7 / 8 about ( 0.8 )
Del