Assuming a traditional war, 40% of VPs is plenty. I actually would have set it lower, but was worried about winning by raids. Apparently I was not worried enough.
My concept was that, yes, winning by raid would be possible, but only if you were able to build up such a strong raiding force, and the other nations weren't defending. I figured any nation powerful enough to seriously mount such an attempt would either 1) already be in a war, sapping their strength, and encouraging their enemies to defend, or 2) be opposed by a similarly powerful peaceful nation, which is more of an arms race situation. I think that would be fine also. But, everyone left Ironhawk in peace, while all the other strong nations got into wars. This is the most important part, more important than victory point ratios or anything like that. Even a minor war would have been enough to prevent this from happening. He was barely able to mount this operation as it was.
I think it boils down to: No reasonable VP percentage will prevent win-by-raid if the VPs are undefended and a player with the wherewithal is left alone to do it. Ironhawk captured enough VPs to meet a 60% threshold, which would have met even Calmon's high suggestion at game start. The question is only whether the defenses get built in time, because they will have to be built eventually.
I'll go along with calmon on one other point: home province VPs means lower VP threshold needed to win, and vice versa.