The information given below was taken from:
The War of the Ring, The History of Middle-earth Volume XIII
The Taming of Smčagol, Footnote #11
When Frodo and Sam come to the edge of the cliff in the EmynMuil. Frodo estimates that it is 18 fathoms to the bottom. After Frodo has climbed down to the point that he cannot go down and cannot return up. Frodo wishes that they had had some rope. Now Sam remembers that he had a coil of rope from the Elves of Lorien.
When Sam measures out the rope he counted out the lenght of the rope in ells. Now if you were like me when I first read this you probably said 'What the heck are ells?'.
From my understanding of Footnote 11. There is a measure called an English-ell which is equal to 45 inches. And what Tolkien first wrote was called a hobbit-ell, thinking that it would be somewhat smaller than an English-ell. But as he wanted he lenght of the rope to be just enough for the job at hand. Tolkein just called the measurement an ell, with the understanding that that was 45 inches.
Sam estimated that he had thirty ells of rope more or less.
And with the understanding that 18 fathoms is 108 feet and 30 ells would come to 112 feet and 6 inches. And considering the the knot and the loop around the stump. They were cutting it very fine.
ells: a traditional unit of length used primarily for measuring cloth. In the English system, one ell equals 20 nails, 45 inches, or 1.25 yards; in metric terms, an English ell equals exactly 1.143 meters. The word comes from the Latin ulna, which originally meant the elbow and is now the name of the bone on the outside of the forearm.
Probably the ell originated through a custom of measuring lengths of cloth using two forearms, with the hands touching or overlapping.