Printing Question - Rephrased

[Len Morgan]

I'm still having trouble printing and my last plea went unanswered so 
I'll try again.

I know that there are 72 points to an inch when I print but I don't know 
how to scale this to dpi on a printer or dpi on the screen for that 
matter.  I'm trying to print ID Badges with a photo on them and I've 
used the example from the video but it seems that trial and error is the 
only way I'm going to get things to fit properly.  What I'd like to know 
is how to map pixels (which RR uses) to physical distances on the paper 
so I can, for example, put the photo in the upper right hand corner of a 
2.125 x 3.375 inch plastic card.

If I do the trial and error once (when my printer is set at 600 dpi, 
aren't the dimensions going to be 2 times (4 times?) bigger if I use a 
300 dpi printer?  Can I get to the printer attributes to determine the 
dpi the printer is going to use and then calculate some constants for 
scaling my card to fit?

Any help would be greatly appreciated even if it was just a pointer to 
somewhere in the documentation.

Thanks!

Len Morgan