24XX provides graduated success. But there's no built in degrees of difficulty. Every challenge has the same odds based solely on the character's skill die.

Given apt adjudication of "this can't happen," "this automatically happens," or "this happens only if you make a plan and complete its steps," that's not really a problem. Especially by making a near-impossible task require multiple steps, it's possible to fine tune the probability of success or failure.

But for one-off cases where you want unskilled characters to have a bit more risk, just -1 from the roll. It won't affect the very skilled very much. It will mean a lot to those rolling d4 or d6.

You might consider giving the players +1 for some cases, but I'd argue that's already covered by the bonus d6 for "helped by circumstances." It's the "harder than usual but not impossible" where that -1 thumb on the odds could come handy.

Oh - so what's the point of this post? I added to RUNNING THE GAME, in Magog: "For hard spots, have the players roll -1."