Well... 1st off here's some info for you to peek at in a minute.......
http://stratics.com/community/threads/reforging-how-to.264020/Oak Runic Saw can go as high as 80% elemental Damage when you simply reforge with just the exquisite category.
If you choose powerful + Exquisite an Oak Runic Saw can now hit 100% Elemental Damage, but those maximum numbers won't happen every time.
Your Basically looking at a 1 in 7 chance to roll elemental damage... following by some percentage chance that it's 100% intensity Maybe 1 in 3 if I had to guess.... Then you have a 1 in 6 chance that it's the Elemental damage you were looking for (physical / fire / cold / poison / energy / chaos). So your basically looking at 1/7 x 1/6 x 1/3 =
1 / 126 or so for your odds of reforging that specific elemental Dmg Wep. If your doing a powerful Exquisite Reforge with a Oak Runic Saw that's 3 charges per Reforge I think so 126 x 3 = 378 Oak Runic Saw Charges will likely produce 1 SPECIFIC elemental weapon... It's actually alot better if your just stashing any 100% weapons you make though.. heh. Anyway you get the idea.