Is there a minimum depth the channel between the tiles should be cleared to so that grout will not discolor with any mastic that may have squeezed into the channel during tile installation? I'm having a shower stall regrouted because the existing grout is discolored in spots by what we're assuming is too much mastic in the grout channel. The builder and I are at odds as to how deep he should clear the channel. He's wanting two-thirds the thickness of the tile and I'm saying the thickness of the tile plus a half again. Would appreciate any thoughts.