Currently trying to figure out how to get my magic system to have reasonable difficulty across the board.
The system runs on attribute + skill + 3d6, with both attributes and skills being 1 to 10. Magic comes in 6 difficulty levels and 10 power levels: the maximum level of power is limited by the skill in a particular kind of magic. You can buy up to 3 specializations at levels 3, 6 and 9 and 3 masteries at 4, 7 and 10 that apply to particular ways of using spells, so you can have up to a +6 bonus at level 10.
I got two possible ways of doings this going on. One is (3D)+P+10, with D being difficulty and P being power. This means the lowest roll at 1 difficulty and power being 14; each increase of power raises difficulty by 1 and an increase of difficulty adds 3. This means that at low levels you have a decent shot at making the roll, but at higher levels it's all but trivial, with difficulty 4 magic at power level 10 requiring you to roll a 12 without taking a specialization or anything. But the highest difficulty of magic becomes all but impossible to cast: you'll need a nat 18 to cast difficulty 3 at level 1, difficulty 4 at level 3, difficulty 5 at level 5 and difficulty 6 at level 7.
The other way to do this is (3P)+(2D). This means at the lowest levels magic is very easy to cast (starts at 5, with each power increase raises the target with 3 and each difficulty increase raises the target by 2), but magic becoming quite a lot more difficult. This means that 10P 6D magic takes a 42, meaning you have to roll a 16 or more, which has a chance of 10 in 216, which is next to nothing. I might add other ways to make actually casting magic easier (maybe something with magic implements or something), but I don't know about this just yet.
The mana system on the other hand will be a lot simpler: your mana points are your Magic attribute times your Attunement skill (which is pretty much for mana only) times three, meaning you have a total of up to 300 points to spend. If you want to restore your points you gotta take a good night's rest.