I honestly think this is one of the more challenging parts of game design. Also, it is impossible to give you exact numbers because it all depends on your damage formulas. I have no idea how much a +1 in strength affects the damage, so I can't tell you what the exact numbers need to be.
So, I can't give you a magic formula but I can give you some tips.
1. Think of how hard you want your game to be. Do you want people to grind while watching anime or do you want every fight to be a struggle? If you want the NPC's to be challenging, it is probably better to make the exp rewards higher. On the other side, if you want people to grind to the next level in your game, it is better to have weaker enemies that give less experience points.
2. Work with larger numbers (at first). It is easier to fine tune your values and balance the game with larger numbers. If the max damage is 10, lowering the damage with 1 point is a 10% difference. With the max damage 100, this is only a 1% difference.
3. Split. Start with (for example) levels 1 - 10. You are in the (pre) Alpha stage, you most likely won't have any content after the first few quests anyways. Make sure that your existing spells and NPC's work and are balanced properly.
4. Slowly introduce new NPC's and Spells. Don't try to balance 50 spells and 200 NPC's at once. Start with like 5 spells and NPC's. Once you have those down, you go on to the next batch.
5. Same as the NPC's and Spells, slowly introduce more complex damage formula's. Don't start with 20 values that all affect each other. Start simple and slowly introduce more to the damage formula so you know exactly what affects what and what you need to change.
6. Test, test, test. This is what an Alpha or Beta version is for. Get people to play and try to break your game. If some class or combination is overly OP, just nerf it. If an NPC is easily beaten but gives too much EXP. Make him stronger or lower the EXP gain from this NPC.
Now these are just some tips from the top of my head. Personally, I will introduce a battle tech demo. Before the actual story and whatnot I want to work out the combat and power of spells by giving players max level and all the spells in the game for free. That way they can play around with creating builds and trying to break my combat system. That way I can slowly introduce spells, NPC's and see how they work + players can start in PvP and I can see if any build or class is overly OP.
Now for values, I would imagine that a player must be about 1,5 times stronger than an NPC their own level. But that is purely a guess , but it might be a good starting point.
I hope this helps!