I was giving some thought to making a simple RPG; ideally an old-school JRPG for all intents and purposes.

Developing the math behind the combat systems and the level systems is going to be a bit tricky, and I'm wondering if there are any good sources, guides, tutorials, or other materials that give some good guidance as to how a solid RPG system can be created.

It's easy to identify the basics. Strength to increase attack, defense to reduce damage, agility, experience, blah blah blah.

But when the rubber hits the road, it's hard to find numbers that work very well across a wide range. What kind of equation should one use to calculate the actual damage, just between the strength and defense variables? I recall playing with an RPG for the playstation, and I found that if I gave the enemies defense values like what the player has, the player did absolutely pitiful amounts of damage. Thus I was prompted to keep the defense values low. But doing that negates the effectiveness of spells and status-afflictions that reduce/enhance someone's defense, because now the defense is already low to begin with.

And altogether, it was just too easy to set someone's stats to make combat impossible if your players are at the wrong levels. If you are too weak for some monsters, you can't do any damage to them at all. I'm not saying it would be a hard and arduous fight that requires a lot of planning, I'm saying you can't even touch those monsters. Then after getting one new piece of equipment or gaining just one level, suddenly they are possible.

I don't even know what equations they were using, but it was hard to keep those numbers in the butter-zone, and there being a butter zone was a problem as well.

So I'm wondering what sort of formulas I can follow to make combat reasonable -even if extremely difficult- when combatants are outside of that butter-zone.

Or at least a good understanding of where that butter-zone is so I can reasonably project good numbers instead of spending all my development doing trial-and-error.

And going along with that is the leveling system. How much experience should it take to get to a new level, and how do I make sure that the player fights monsters at an appropriate level, instead of finding it better to keep fighting weaker monsters from an older area? How much does a player's stats increase with each level to make their leveling be useful, but not also cause huge rifts in stats that make (for example) a mage become comparatively weaker each level, because the attack power is too low?
And along with the leveling is setting up weapons and armor so that getting new equipment is helpful, but not a complete game-changer.

There's a lot of things that go into this to make these systems work. I'm pretty sure someone somewhere has a good set of patterns to follow or general advice or something. But I couldn't find it on a google search. (Maybe I'm using the wrong keywords; I don't know.) I figure there are a lot of different ways to approach this, and the exact numbers you'd want are going to depend heavily on things like how many battles you want you player to fight to gain a level, how great of a difference do you want one level to make, how long do you want a battle to last, how capable will the player be against an low-level enemy, and of course, how will the unique system you create for your game effect these variables. So I don't expect there to be one single system to follow, but it would be very useful to get an idea of how to approach these things.

I think it less about math, and more about keeping the balance between boring and frustrating.

http://uxmag.com/articles/the-good-tough-problem

The challenge will be to maintain this rise in difficulty so beginners don't get frustrated and hard-core gamers don't get bored. There may not be any equations that you can come up with to make this work.

BedderDanu touched up on this, but it didn't dawn on me until last night.

As I had mentioned in my first post, I was worried about creating a system where people with attack values too low could not do any effective damage against an enemy.

I've seen this in a number of RPGs, actually. For many levels a character can't do more than two points of damage. Then they make a sharp increase, suddenly able to do small but noticeable amounts of damage, and then able to inflict the expected amount of damage.

I never liked this, because it doesn't make fighting a powerful foe challenging, but completely impossible.

But this problem is worked around by simply having the defense modify the attack value by a percentage

I want to phrase this more explicitly.

You perform all the desired calculations for damage, and then at the last step the defense shaves off a percentage of that value. For example, if the maximum possible defense was 100 (or rather, 99, but you calculate it as 100) then a defense of 15 would reduce all attacks by 15%. If the maximum possible defense was 256 (technically 255) then a defense of 128 would reduce all attacks by 50%.

That way, even if you attacked a high-level monster you would still deal some damage. Furthermore, the defense of the monsters themselves is relatively arbitrary. A level-one monster could have a defense of 50% or 10% and would still be just as accessible to a first-level player.