What % grain needed in potato beer to convert starches

I'm thinking about making a sweet potato or yam beer and want to get goofy with it. I want to use just enough grain to convert the starches in the potato/sweet potato/yam. What % of the grain bill would be absolutely necessary to convert the starches. I don't mind a 2 hour mash either and I predict there will be a few rice hulls. I plan on shredding the yams then tossing them in the oven for 75min at 425*F and a few are staying on the baking sheet for another 10 min top rack on broil... probly 5% of them, LOL! I think I'll go easy on the hops too.

Since we are talking enzymes, it's a matter of time & concentration. Enzymes are not consumed by the conversion process, but I wouldn't go below 1/4 of the bill.

One really old recipe, but he uses more grain than potato. http://brewery.org/library/Potato.html

Sweet potatoes & yams can probably be treated just like pumpkin. All three are more water than starch.