Vintage pattern thread yardages

[kathome]

Does anyone know if a chart of yardages of threads called for in vintage patterns exists?

 

The pattern that I want to begin is for a centerpiece or a tablecloth (if one is so ambitious) and for the pattern calls for: :

 

J&P Coats or Clarks ONT Best Six Cord Mercerized Crochet Cotton, size 30: Small ball: J & P Coats—7 balls of White or Ecru, or any color; or Clarks ONT—11 balls of White or Ecru, or 13 balls of any color. Steel crochet hook No.10.

 

My eyes are getting a bit too old for size 30, and so I would use Cebelia 20 which has 405 yards per ball, so how many balls shall I buy?????

 

Thanks