I hope I am answering your question correctly. The formula is not a 50/50 ratio in regards to rushing and receiving. It is based on a per touch basis. If there was a RB who caught 10 passes to every 1 rush his rating would most likely be higher than a RB who caught 1 pass for every 10 rushes, solely based on yards per touch. Again, TDs per touch are the most important factor in this rating, followed by yards per touch, then fumbles per touch.
I do plan on making a system for receivers however I can't use the same parameters as a RB as a WR most often gets more yards per touch than a RB. As of now I am looking at numbers to determine what the "best" production is. That formula will again follow the 0-158.3 rating system, an attempt to equate a players value regardless position.
The criteria I am using for the WR Rating is yards per touch, TDs per touch, fumbles per touch, and drops per targeted pass. The last stat will be the hardest to quantify but ProFootballFocus.com can provide the necessary information.
It will take some time, but my goal is to create a rating system (within the QB rating model) for every position. Again, this is a stat, it helps in evaluating a player but is not a complete determination of a players value.
Please keep the questions and feedback coming, I'm a numbers geek and love doing this kind of thing.