'Rick K' here is one way to estimate how much of a curtain type of sheet wood you would have to purchase for the required ribs of a sailplane project:

Generally there are certain ribs that are the same such as ribs used in the center section of a sailplane (not the tip sections). You can make a copy of the ribs onto tracing paper (or whatever) and cut out maybe four of these from paper. Then draw an outline of the sheet balsa you are considering purchasing. Lets assume you find a 'real deal' on sheets that are 3 inches wide and 32 inches long (not three feet). Draw that outline on a sheet of paper (you might have to tape pieces of paper together to create the proper 3 x 32 inch size). Then lay out the paper ribs on the paper 'wood sheet' and figure out the best way to keep the grain running correctly for each rib and yet waste the least amount of wood. Once you have determined how many ribs you can cut out of ONE sheet, and you know how many ribs you need in total (plus add a couple of center ribs for good measure), you can estimate how many full wood sheets of that size you need for the center ribs.

The tip panels generally are tapered so you will be able to fit more tip ribs on one sheet of wood. But you do not want to underestimate. You would be able to make a good guess on how many tip ribs you can cut out of a wood sheet based on the center rib calculations you made above. You will probably end up with some extra wood but that is better than having too little wood.

It would be a good idea to start with a small sailplane to get a feel for what is demanded from you in starting and finishing a hand cut project. I suggest you start with something you 'really want' when you are finished as this helps to keep you motivated and gets you past the more demanding work.