There's not really a standard since scopes aren't standard in design or mounting position, but if the power ring and/or ocular bell clear the hammer in the right place, mediums will work, it just depends. I use Burris Signature Zees, mediums most of the time, but some scopes don't allow them, need to go high. I have long arms, so I can mount a scope forward enough and still have a good view thru the scope, but if it needs to be mounted with the hammer right at the power ring or ocular bell, highs are probably the only choice unless you bob the hammer off a bit as shown in the FAQs.
Tim