Acoustic TReatments - What, How much, & Where?

Hello all you room treatmentfiles.

Being at the point where I probably should have begun, now, I want to 'treat' the room, acoustically. Economic reasons say I might have to go by way of ‘foam’ stick ups on the wall, but I’d really rather not. The desired alternative, however, is panels, and they are mighty pricey. OK.

Reading over the online info at various panel makers websites, (ASC, RPG, GIK, etc), some panels have different ranges of absorbtion, reflection, etc. according to the makers.

Q: How do you know just the amount of absorbtion, or diffusion to incorporate into the room via panels, traps, etc.?

Is there some Rosetta stone that will decrypt this for me… or is it all “trial & ear-ror”?

GIK panels seem the most feasible cost alternative, presently.

Any suggestions or related experiences in regard to sources for, or treatments of, will be more than a welcome thing.

Thank you