I'm not even sure how Expert Grails go since I haven't done any in like six months, but it used to vary WILDLY from event to event. Sometimes it was guaranteed 5, sometimes it was 1-5, sometimes 3-5. Standard has been a pretty solid 1-3 with average of 2 throughout the history of Kamihime though. And since it has a fraction of HP of Expert, it's usually been more Grails per hour than Experts, and sometimes even more Grails per AP. Though especially when Experts are guaranteed 5, stronger players will absolutely want to run those.
Point is, Standards are perfectly fine for Grail farming simply because it requires basically no powerlevel and it's not even bad in terms of AP/Grail and Time/Grail ratio. Because I have no idea if we're talking to a Rank20 player, or Rank50, or Rank 100, or Rank150, we might as well go by the Standard standard.