How to go about reducing sugar in a butter cake recipe? Recently I made some cupcakes that turned out mostly great. However, even other people than me – I have an aversion towards overly "sugary" desserts – said they were fairly sweet, and for me it seemed to obscure the other flavors. 

(The other flavours being vanilla or rose in the batter. You could certainly smell them so I don't think it was just that they evaporated while baking, and I use a fairly heavy hand with those ingredients for baked goods.)

Is there any rule-of-thumb on what the "correct" ratio of sugar to other ingredients in a cake recipe is so I could tell if the recipe was using too much? (The recipe was: cream butter and sugar, add flour and baking powder, add eggs, add milk, stir until just combined, bake.) Or how far I could reduce sugar from the standard recipe without negatively affecting texture / moisture? And is it necessary to adjust any of the other ingredient amounts along with the sugar?
According to Shirley Corriher, as reported in Fine Cooking, the sugar should weigh slightly more than the flour:

The sugar should weigh the same as, or slightly more than, the flour.
Remember that this is weight, not volume. A cup of sugar weighs about
7 ounces, and a cup of all-purpose flour weighs about 4-1/2 ounces.
So, if we're building a recipe with 1 cup sugar, we'll need about
1-1/2 cups flour (about 6-3/4 ounces).

Sugar plays a very important role in creating the structure and bite of cakes, and also helps to retain moisture and inhibit spoiling, so you cannot simply reduce the quantity without limit to control the sweetness.
That being said, most recipes have a certain amount of tolerance. You can probably reduce the sugar by about 20 to 25% without completely altering the nature of the recipe, although the crumb may suffer a little.

Vanilla and rose are delicate flavors. If you are making a yellow cake with butter, and egg yolks, they may be competing and masking the flavor.
Using a white cake base may allow these flavors to show through more clearly.