Basic Muffin Formula Range/Percentage

Michael Ruhlman, author of the book Ratio, suggests 2 parts flour, 2 parts liquid, 1 part fat, and 1 part egg (by weight). In the section on batters, provides a recipe which includes an additional 1 part sugar (for a basic sweet muffin), but he also suggests several variations on the basic recipe.

In general, Ruhlman's book is a solid reference whenever one is tempted to ask "are their basic percentages or standard formulae for ___?" The cover of the book gives a lot of basic ratios for cooking:

The most prototypical muffins I know are made from a pound cake batter, using the muffin method (mix all at once). That would be a ratio of 1:1:1:1 (flour, sugar, butter, eggs).

If you subscribe to the theory that the muffin is a form of not-so-fancy cake, then it is worth to take a look at the ratio range suggested for such cakes. Shirley Corriher calls them "regular shortened cakes" and offers the following formulas for successful experimenting:

1. The weight of the sugar should be equal to or less than the weight of the flour 
2. The weight of the eggs should be equal to or greater than the weight of the fat 
3. The weight of the liquids (eggs and milk) should equal the weight of the flour. 

She also recommends "1 to 1 1/4 teaspoons baking powder for each cup of flour, or 1/4 baking soda for each cup of flour".

Of course, "muffin" is not a well-defined category, and has quite some overlap with "cup cake". For some people, the above will be too cakey to count as a muffin, while others will bake cupcakes that are richer than it and still call them muffins.

Also, muffins often include mixins, such as nuts, dried fruit, or chocolate chips. These are obviously not part of the basic cake formula above, but in my personal experience, the total amount of mixins should be no more than 20% of the total weight of the batter (=the sum of ingredients other than mixins).