The standard size for a twin is 39" x 75" without extra drop for the sides and end. 2 1/4 yds would cover that, depending upon the blocks, borders, etc.. that you have. Your figures seem a little low to me, but if they came from a particular pattern, they may be right.
My advice is to get more, you can always have some for your stash, but you can't strech what you have.
Lanny