A difficult question. It would depend on the thickness of the yarn, how high the twist and if it is to be plied. I am not sure that there is a formula that will accomodate three variables. You might use a weight guesstimate of the finished item, and use that as a criterion for purchace, keeping in mind that too much roving is better than too little.

I've been told by those who sell roving, that 1LB will do a sweater, although I usually get 1 1/2 as my husband and I are both big folks - haven't run out yet, but as madammim [above post] says, it's a tough question with a lot of variables. As she said, too much is better than too little!

I don't know how to figure roving and figure twist and weight of the yarn....but 1 pound of yarn would be equal to 4 4oz skein of wool....and if I remember correctly that would be about 380 yards of worsted weight wool yarn.
Linda C