95%.
However, this is just an abstract, theoretical answer. I generally prefer combat systems where the results are botch/critical fail/miss (5%), regular success (90%) and critical success (5%), except when targeting something specifically twinked for evasion. This is closer to the console RPG standard where the 'attack roll' is little more than a formality and attack effectiveness is wrapped up in hit point damage.
70% sounds fine.
Now, because the orc in Orc and Pie is a solitary monster, in fact the only monster in the entire adventure, I'd want him to be seriously NASTY. Say, a hit rate lower than or at least no higher than a PC's, but with a Celerity effect that gives him at least two actions per round (ideally, these would be spread out over the round), hp greater than the total of the party, and damage sufficient to drop a PC in two attacks.