If you sometimes feel as if you are all thumbs when it comes to business, Tom Parker has a book for you. Called Rules of Thumb (Houghton Mifflin Co., $5.95), it is a compendium of observations, judgments, and nuggets of accepted wisdom, on everything from planning a federal budget (a 1% rise in the unemployment rate cuts revenues by $12 billion a year) to measuring mice (there are about 250 mice to the gallon).

Among the homilies, drawn from a wide range of sources, are some general rules for business life. When writing an ad, for example, Parker's book says the rule of thumb is to keep your sentences to fewer than 12 words. For brainstorming sessions, the rule is to have at least 5 people and preferably 12.

Parker also includes some words to the wise, which don't really qualify as "rules of thumb" but still may be worth heeding. Rule 660 warns that a new venture probably won't fly if the concept can't be put into one sentence, and rule 661 urges that any new product have at least three advantages over its competition.

And, of course, there is a whole raft of 80-20 rules. Rule 718, for one, says that 20% of a product line generally produces 80% of the profit.

But perhaps the one that best sums up the whole cracker-barrel philosophy is number 896: When in doubt, don't.