The market is going to dictate what the typical percentage of purchase price you can reasonably get per month. In the multifamily space we talk about cap rates, which are somewhat similar in idea. You are just getting an idea for how much you have to put in (if paid all cash) in order to get a certain monthly rent. The 2% rule doesn't take into account any expenses you have and the demand for your area, so certain areas of the country will have different returns and it wouldn't be prudent to apply a rule everywhere.
You are also talking about Single Family Residences. These are usually purchased by residential homeowners, with different likes and needs from investors. They are much more numerous so they will sway the market. If their demand goes up, then the prices go up and your percentage return goes down.
Figure out your expenses for a typical property, the average rents in the area, and work from there to find how much you can reasonably pay for a property. Do this enough times and you will find how much you can pay for properties in your area. This percentage will be your x% rule. You then use that to quickly weed out properties before doing more diligence on them. Remember, just because it passes a rule such as this doesn't mean it is a great deal, it just means you can devote more energy to evaluating it further.