Right.....lets do this in 2 parts....web and print.
A: For Web.
1. Forget all about resolution, ppi, dpi, mfi, mph, tnt.....ya get the point?
2. Pixels are the main thing for web....an image will be the same on a web page regardless of whether its 72ppi, 1ppi or 3000ppi.....makes no difference.....forget it.
B. For Print.
1. You have a finite number of pixels....in this case, 480x720px and setting a resolution of 240ppi gives you a print size of 2"x3"...so far so good.
C. (Ok, 3 parts)
1. I'm now assuming that you want to do 'something' with 'B' so that the 'pendant' ends up being a 1.5" diameter circle when printed on the page...is that correct?
Is that the problem?
How to re-size so that the pendant is that size but the overall image stays 2"x3"....ie, cropped to 2"x3"....?
I may have totally lost the plot here but then again I'm not actually convinced I fully understood the question to begin with...sorry.
If you can just clarify exactly what it is you are trying to do I'm sure we crack this before dawn.
Regards.
MrTom.