I've seen some excel sheets on the web that will give you the answer in a jiffy. Hunt around for them or ask a good welders forum - there are pros who crunch the numbers for you and spew out the answers online!
i remember this formula: P= WL / E where:
P is the Pounds of electrode or wire that is required
W is the Weight per foot of weld metal
L is the Length of weld in feet and
E is the Deposition efficiency