My personal favorite dpi script is ... drum roll .. my calculator.
[Size in Inches] TIMES [Resolution per Inch] (i.e. 300 dpi, 450 dpi, etc.)
EQUALS the number of pixels the image needs to be rendered.
Example: Image needs to be 26.5" in width, to be printed @ 450 dpi:
26" x 450 dpi = 11925 pixels
If you work in inches, but for those that don't it's more hassle.
Pixels per cm or pixels per inch: pixel-dimension is derived by exactly the same arithmetic. So why would one of them be a hassle? Sorry, I just don't get it.
300 pixels per inch is the same as 118.11 per cm. 450 per inch = 177.165 per cm. After that, just substitute numbers and calculate.