I don't have a Riser but a lot of the 2m gliders like that would have the outer dihedral break in the 10degree range.

I think the Oly 2m had 10degrees out there. The standard Oly has 8degrees however.

I never built but one or two kits and didn't keep those plans so I can't look anything up. However, that 10degrees was common enough that I made myself a building board with that for the outer dihedral break. So I feel confident in suggesting that 10degrees will work well.