COMDASC, The two sliders in the Print Module are best left alone, i.e. best not used at all. The reason for this is very simple: The adjustments with the sliders are not visible in Lr, so you have no feedback of how much your print will be affected.
The correct way to match a print to what you see on screen (or better worded: to match your screen to the print) is to calibrate the monitor and set it to the correct value for brightness (intensity). The tonality (brightness/darkness) and the colors of a digital image are determined by the color numbers. For each pixel there are three numerical values that determine the tone and color of this pixel. The printer prints the image according to the color numbers. But uncalibrated monitors cannot be trusted to display the image according to the color numbers. Most uncalibrated monitors are to bright (because they are set for brightly lit offices) and they have a color bias (too much red, or too much blue, or too much green). Color calibration is done with a piece of hardware called a spectrometer and accompanying software. Brand names are Spyder, ColorMunki, GretaghMacbeth. The software lets you select a target for White Balance (6,500 K is the industry standard) and for brightness (intensity) of the monitor. Select a brightness of about 110 cd/m2 to match the brightness of your monitor to your print.
And when your monitor is calibrated and set to correct brightness you don't have to adjust for the various media. Then it's WYSIWYG - What You See IS What You GET in print.