Basic rule of thumb is 50/50 hard and soft reflection on each surface.
Easiest way to handle the above and still get reasonable acoustic results is to put the measured and calculated swuare footage of absorption in the CENTER of the surface with the hard part making an equal sized border around the soft.
There are 6 surfaces in the average room, not four. Don't neglect the floor and ceiling. Floor can be handled with a throw rug, again centered with the hard floor border around the edges, although I wouldn't sweat that in a wall-t0-wall carpeted room as there will still be plenty of hard reflection on the other 5 surfaces.
Ceiling can sometimes be problematic if hard throughout and painted. There are acoustic absorbing paints readily available that can help with the ceiling, as well as ceiling tiles if the ping or pong is still evident.
In the home studio environment, lots can be done without resorting to attempts to make the room look like a modern recording studio. For example, a wall covered with bookshelves and having different size and amount of books on all shelves can work very well as a standin for Helmholtz filters. A large upholstered couch can absorb quite a bit of acoustic energy that might otherwise be reflected. Pulling that same couch out from the wall a foor or so can create a crude Bass Trap situation if you find that to be a problem in your home studio.
I know of one enterprising young fellow who produced his entire first CD in his bedroom. He used the mattress from the bed in the upright to form his ersatz vocal booth, the same SM-58 mic he had to do live performance with - and his vocals sounded very good indeed. When it came time to record his electric guitar, he decided he preferred his familiar amp'd sound to any simulation plugins, so he laid the bed mattress on its side in front of the guitar amp which was placed facing outwards in a corner of the room. Yep, same mic, SM-58.
When tracking like this, it is important to do a lot of test takes, listening carefully to the result and then governing any changes accordingly, such as mic placement (can be extremely important, sometimes moving the mic a bit closer or further away changes things a lot), amp settings (too much low end feels good to the average guitarist, but muddies up a track at playback), gain staging (loudest is not always "bestest"), and, for most guitarists who have not had a lot of studio experience, BACK OFF ON THE DISTORTION SETTINGS as a little bit goes a long long way when the track is played back with the others.
--Mac