There is some advise in how to approach this kind of truss in the "Timber framer's workshop" ( ordered it of this web page.

Taking your picture you could build that arch with four pieces that are slightly curved. Just add another joint at the top of the arch. I would also add a king post since there is going to be a lot of thrust on the rafter tie.
You could use a semi circular arch if it happens to fit inside your roof system. In most cases it will not so an elliptical arch is a lot more adaptable.
How big is your work shop? You could simply lay out the roof/truss system you are trying to build on the floor, to scale and then find the arch. Then you simply transfer the layout onto the timbers.
Hope this helps some.