BATTLE SYSTEM BALANCE

Hey all. I've been doing quite a bit of research and reading but I haven't found a solution to a particular issue:

I understand that your typical RPG battle system is a big dice roll. A game of numbers and formulas. Is there a particular formula set out there or do most people
just play with things to see what does and doesn't work.

I ask because I want to be sure that things are balanced and then from there and THEN I go in and start tweaking to make things interesting or challenging. Should there be certain numbers I keep in mind?

For example. should I approach things based on levels? Where Character A possesses stats X,Y,Z at level 1 so I should plan enemies and accessories based on the most likely levels a character will have by the time they reach those enemies or weapons/armors. Should I always think of Character A, in that scenario, as potentially always reaching a base level progression (say level 5) by a point or should I plan a bit under (assume that they'll be level 3 instead of 5) or a bit over (assume they've been grinding and reached level 7 rather than 5)

I'm not planning a fancy battle system. Just something typical with enemies and characters possessing different attributes and such to throw in a bit of strategy flavor (I know some people aren't into that kind of thing, and that that's okay).

Any pointers from veterans or "those in the know"?