Note that I also posted this to Monte Cook's board, in hopes of attracting his attention--but I'm interested in how anybody does this.
Okay. This has been touched on elsewhere (problems with d20 thread, whether you use -1 sanity per rank of Mythos knowledge of -5) but I'm going to bring it up explicitly here because I don't find where it has been discussed in exactly this way. (If this has been discussed extensively and a consensus reached, please point me there, and I apologize for wasting your time.)
What difficulties do you use for Mythos skill checks? Bear in mind that the rulebook has no examples and no guidelines about when one uses Mythos skill anyway. The implication is that the difficulties for the Mythos skill are on the same 1-30 scale as other skills. (But Mythos skill isn't like other skills! That's why we need more guidance! :0 )
BRP uses a d% for skill difficulties, where the player rolls under the value (some modification by the Keeper is allowed...+10% or -10%, for example). d20 uses a d20 scale where the player rolls over a difficulty value, usually rated from 1-30.
In keeping with this, the ranks of Mythos knowledge granted by tomes are approximately 1/5th of the ranks granted by those tomes in the BRP version of the game. This makes sense; a d20 has 1/5th the number of probable outcomes as a d%.
The mechanical equivalent of "roll under" in d20 is to set the difficulty value at 20--that is, it provides approximately the same probabilities of success. To preserve the feel for the mythos knowledge being hard to gain and use, a simple solution would be to arbitrarily set the DC for Mythos skill checks at 20.
Except: There is no limit to the number of ranks of Mythos Skill a character can get.
A character can have 30 ranks of Mythos knowledge. (Sure, his or her maximum Sanity is now 70, but that's not much of an impediment. Maximum Sanity has not been significant in my BRP games and I don't expect it to be significant in my d20 games. It simply means that after a certain point characters may not see an improvement in Sanity when they level up. Though by the time a BRP character has the equivalent of 30 ranks of Mythos knowledge, he or she has been a screaming lunatic for a while: 99 Sanity - (30 ranks * 5) = -51 SAN.)
I am trying to reconcile these two things: standard difficulty levels versus no limit to the amount of Mythos skill.
The options, as I see them, are to set DCs differently than for other skills, or to cap Mythos skill at some point.
What do other people do? Does it simply not happen that characters get 15-30 ranks of Mythos knowledge?
Or do you use skill difficulties in a 1-100 range?
I am currently planning on house-ruling this, so that a character cannot have more than a certain number of ranks of Mythos skill, though I have yet to decide on the limit -- perhaps 25, which will give even the most powerful Mythos sorceror a chance not to know something obscure (DC 30, roll of 1-4); or perhaps the character's Intelligence score.
I am also planning to make Mythos skill the appropriate skill for Mythos spells, rather than Spellcraft, which will work for non-Mythos magic.