Each installation must be considered individually. There are, however, a few general guidelines to follow in choosing the type of trowel to use for a particular installation. Small sized, smooth-backed tiles such as ceramic mosaics do not require as much setting material for bonding as do larger tiles with patterned or uneven backs. V-notched trowels are usually used with mastic adhesives which are not sanded and can be readily compressed when tiles are beaten in. Adhesives are limited to walls, residential, and light commercial floors.
Coverage is of the utmost importance.
Generally, the more uneven the back of the tile or the larger the lug on its back, the deeper the notch is required. This is to ensure contact with all points, full coverage and complete contact with the substrate. Set tile while mortar is fresh. To test for this, place your finger in the setting material which has been spread on the substrate. If no material comes off on your finger, it will not bond to the tile, either.