Where to start with numbers and balancing?

Hello all! I have a project that I’ve been working on for a bout 6 months or so now. I’m to the point where I want to get a starting place for all my numbers.

My question is-- What is the best way to approach setting up initial values for numbers such as Health, Damage, etc… Are there any ratios I should adhere to or rules of thumb?

The player characters have a health value and they carry a weapon that has damage, rate of fire, etc, and can be increased by selected stats by the player. Each enemy as its own health value and damage that also increase during game play.

Or is the correct approach just to play, play, and play again until I find a point that is comfortable?