I don't have a 7150. However, you need to open the printer driver settings (Page Setup) from within the LR printmodule and select the borderless option and the correct paper size and orientation. Being back in LR you now can move the margin sliders to 0 and set the cell size to your paper size. You should see the results in the preview.
Your advice was good, but apparently my printer (HP 7510 Photosmart) doesn’t allow the borderless option and LR wouldn’t let me adjust the sliders to zero. It keeps resetting to .12. Thanks anyway for your advice.
LR reads out the minimum borders from the printer driver and does not allow values smaller. This is why it is important to set that option in the pages setup/driver settings before - if the printer supports it.