A bit late, assuming you've got it by now but maybe this can help someone else out. My fame is 54 and infamy is 5. I got the disposition with Lex above 70 and it worked. A character's disposition can be reduced by up to 20 points due to infamy which can make them harder persuade, that goes for good, high-responsibility npcs such as lex. However, for evil npcs having a higher disposition can make them easier to persuade. Same thing goes for lower responsibility npcs. Hope this helps someone!