Increment opacity in an action

How can I record a drop of 15% in opacity in an action?

You can’t, you need to employ Scripting which can automate operations that are based on certain conditions.

If you want to give it a try, paste the following text into a new file in ExtendScript Toolkit (part of Photoshop’s installation, Applications/Utilities/Adobe Utilities/ExtendScript Toolkit CS4 or /Applications/Utilities/Adobe Utilities-CS5/ExtendScript Toolkit CS5) and save it as a jsx-file into Photoshop’s Presets/Scripts-folder.

After restarting Photoshop the Script should be available under File > Scripts and can be assigned a Keyboard Shortcut directly, recorded into an Action, (in CS4 and CS5) be used in a Configurator-Panel or started from ExtendScript Toolkit directly.

// 2012, use it at your own risk;
#target photoshop;
if (app.documents.length > 0) {
var myDocument = app.activeDocument;
var theLayer = myDocument.activeLayer;
if (theLayer.isBackgroundLayer == false) {
var theOpacity = theLayer.opacity;
var newOpacity = theOpacity - 15;
if (newOpacity <= 0) {newOpacity = 0};
theLayer.opacity = newOpacity;
}
};

Well, I would not have thought of that.

By the way, do you do the rotations on Smart Object instances or plain duplicated Layers?

I asked out of concern with regard to image deterioration.

Applying several transformations (rotations in this case) may reduce the quality that one could avoid by working with Smart Objects.

What is the ultimate purpose of the Action anyway, if I may ask?

I know PS is not optimal for logo creation

One has to use what one knows how to use, I guess.

At the moment I'm experimenting with rotating duplicates of an element in order to form a new shape/pattern.

Probably easier done in Illustrator, but if you do it in Photoshop all the more reason to use Smart Objects, because then one can change/update multiple instances in one fell swoop.

For circular arrangements you could try a Script from this thread:

http://forums.adobe.com/message/4195530#4195530