60% would probably be a good starting point. The key though, is to hydrate and mix the dough to the consistency as described in the recipe; ie, sticky, tacky, soft, firm, etc.
"...We have found that due to the delicate nature of the spelt gluten you MUST reduce the mixing time for VITA-SPELT to about 1/2 of what the manufacturer recommends. We recommend no more than 4 to 4.5 Min at a medium speed.
Furthermore, reduce the amount of water you use by 25%..."
I just made a loaf where I substituted half the flour with spelt, but I didn't have to reduce the water. In fact, I had to add a little extra to get to the needed consistency.
60% would probably be a good starting point. The key though, is to hydrate and mix the dough to the consistency as described in the recipe; ie, sticky, tacky, soft, firm, etc.
http://www.purityfoods.com/cooking_resources/hints_and_tips
I just made a loaf where I substituted half the flour with spelt, but I didn't have to reduce the water. In fact, I had to add a little extra to get to the needed consistency.