What is a good system for determining if a character can make an ability check at all?

I'm working on a tabletop RPG system for a game that is high-fantasy but also has a sci-fi component. The game will often have characters from a medieval fantasy setting interacting with characters from the sci-fi setting.<br><br>I'd like each check to have both a difficulty rating (describing how hard the check is) and a **vocational familiarity rating** which describes how familiar with the vocation a person from a certain setting would be.<br><br>For instance, the difficulty rating for a check to "defuse bomb" would be very high, because bombs are difficult to defuse. But the vocational familiarity rating would be very low, because people from a medieval fantasy setting wouldn't even know what a bomb is, let alone how to defuse it. <br><br>Likewise, for an "attempt to smite someone with the power of a deity" check, it would have a very low vocational familiarity rating for a person from the sci-fi setting, because they wouldn't even know that deities are real or how to ask for their help. But the difficulty rating would be very high because smiting people is a difficult check.<br><br>I'd like each character to have an associated "vocational familiarity threshold" that describes their ability to make checks that are unfamiliar to them. Whenever they attempt a check, I'd like to compare the vocational familiarity rating of the check to their vocational familiarity threshold.<br><br>If the vocational familiarity rating of the check is far below their vocational familiarity threshold, I'd like to say that the character is completely unable to make the check. If the vocational familiarity rating of the check is just below their vocational familiarity threshold, I'd like to allow the check but apply a penalty.<br><br>How should I set a vocational familiarity threshold for a character? A simple system is the best one. Ideally, I can avoid an overly elaborate system of skills, vocations, careers, etc. A 0-100 system where a character with a vocational familiarity threshold of 10 is a medieval blacksmith and a character with a vocational familiarity threshold of 90 is a trained astronaut seems perfect.<br><br>How should I apply penalties? Again, a simple formula is best. Should I just subtract their vocational familiarity threshold from the vocational familiarity rating of the check, and make the penalty proportionate to the difference?