Why is it being read wrong?? The way the resolution is displayed in the "image size" dialog box has no effect on it's actual size. If you made it 90 p/cm you should see the equivalent in p/in.
If the problem is that when you action goes to rasterize the PDF, it's entering 90 but the box is set to p/in, just figure out the equivalent in p/in and have it enter that number. The easiest way to figure this out is to make a new file set to 90 p/cm, then go to the image size dialog box. Change the setting to p/in and it should show you the corresponding number.
We used to do this all the time as our scanner was metric, but we were used to working in inches. that's how I know 120 p/cm = 304.8 p/in
