ルール化 means 'rule setting'. Depending on the context, it may be translated as 'codification', 'regularization', and so on.

Turn it into a rule then( apply to the design.)

http://en.wikipedia.org/wiki/Generalization
http://dictionary.reference.com/browse/generalizations


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Note added at 2 hrs (2010-02-27 04:21:19 GMT)
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http://www.abysshr.com/mdklg010.html

From the original blog:
「自分の好きだと思うデザインを収集し、なぜ好きなのか、その法則・ルールを見つけ出して応用すれば好きなデザインが生まれる、というのが佐藤さんの手法だそうです。」
This is talking about the formulation or quantification of the rules first (figuring out what the rules are or ought to be), then applying them for future designs. Most often in Japanese ルール化 is used to mean the establishment or enactment of rules that are already understood (such as making it a rule to wear helmets in a hazardous area), but in this context it is slightly different.
"Formulation" can be substituted by a number of other words: deduction, organization, tabulation, extraction, extrapolation, etc.
In more colloquial terms, the person is trying to "figure out" the basic reasons why he/she likes the design, and using those rules to extrapolate what other designs would be pleasing.

There are many sites that refer to design guidelines or guidelines for designing, etc. and I would tend to use the word "guidelines" since it is about "setting standards."

A guideline is defined in the WordWeb Dictionary as: A detailed plan or explanation to guide you in setting standards or determining a course of action

Please see below website and sentence examples:
http://articles.sitepoint.com/article/design-guidelines

I have compiled a list of general guidelines that you should follow while designing your site. The guidelines below are widely accepted throughout the website design industry, and will help make your page more attractive, and user friendly.

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Note added at 7 hrs (2010-02-27 08:28:42 GMT)
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Oops. :-) I didn't mean be so presumptuous as to put a "5" for confidence....I pressed the wrong button. But, I still am confident in the word "guideline/guidelines."

And we have “scaled up” design methodology to address problems of corporate ... From Design Person… Design originated in handcraft and the decorative arts, ...
www.ideafactory.com/magazine/mag_thenewdesign.asp -