Using Photoshop Input/Output Levels

I have been playing around (playing being the operative word!) with the input/output values of the Levels tool in Photoshop to try and optimize the prints I'm getting from my Epson 4900. By trial, error and guesswork, I'm using 5:247 values which seem to protect the highlights (minimal areas that don't have ink applied on the print) where there is detail in snow or clouds. While I can see that using this approach for the highlights works (i.e. seeing if ink's applied), what approach is best for the deep shadows? I have searched for more information but can't seem to find to much information giving me a more structured approach to what these numbers should be. Are they paper and/or printer and or ink related values?

Any assistance would be greatly appreciated.