Donna:
I understand what you are trying to say, but the answer to your question is in fact that you're starting your tiling wrong.
You see, instead of starting your tiling in a corner (for example, the corner where the top of the tub meets the front wall of the tub enclosure, start your tiling ABOVE that by marking a horizontal line all the way around the tub enclosure some height above the tub.
Say for example, if you're using 6 inch by 6 inch tiles, you'd mark your horizontal line 5 inches above the top of the tub all the way around the tub enclosure.
THEN, you fasten something straight (like a piece of straight wood molding) so the top edge is right along that horizontal line.
You set all your tiles on top of that wood molding first, then remove the wood molding and CUT the very bottom row to fit down to the tub.
If your tiling is going to go down to the floor, then pick your horizontal line elevation so that you get most of the height of a full tile at the floor as well.
NOW, if by doing that, you find that you end up requiring 3/4 inch high tiles at the ceilng, then incorporate a 1 or 2 inch tall row of border tiles in your tiling so that the top tile at the ceiling will be almost a full tile cut at the top to fit up to the ceiling.
Similarily, don't start tiling the long 5 foot wall at the front of the tub or the back of the tub either. Mark a vertical line toward the middle of that wall and position that line so that you have to cut most of a tile to fit up to the front wall and to fit up to the back wall.
In that way, your tiling will look straight regardless of whether or not your tub is set level or not. That's cuz you set your tiling horizontally, and cut the perimeter tiles to fit up to your tub, walls and ceilings, and regardless of how crooked those lines are, your tiling will still fit up to them to make the installation look good.
If you start setting tile on top of your tub, then if the tub isn't level, all your vertical grout lines will lean one way or the other, with the result that you'll end up with a progressively widening grout joint in the corner as you tile up to the ceiling. By doing it my way, you avoid all those problems.
Also, if you are using rectangular tiles, set out two rows of tiles along the floor with spacers between them set both end to end and side to side. You can then take measurements off those rows to determine exactly where the edges of your tiles will end up once set.
I find that 1 1/2 inch wide by 1/4 inch thick wood moldings are darn near ideal for setting tiles on top of. I use double sided mounting tape to stick them to the walls, then just set the bottom two rows of tile first. Once the adhesive holding that bottom two rows sets up, then those two rows will support all the subsequent rows set above them. You just need the wood to stop the tile from sliding down the wall as the adhesive sets up.
Also, when I tile, I NEVER use mastic. I use a thin set mortar. I measure and mark off a rectangular area of wall about 6 to 8 square feet and then use 2 inch wide masking tape to mask off the edges of that area. Then I spread thin set on the wall with a 1/4" V notch trowel. Then, I simply back butter each 6X8 inch tile with thin set mortar before pressing it into place. That way, even if the mortar on the wall skins over, the moisture from the wet mortar on the back of the tile will re-activate it to give me a bond that's just as strong as if both mortars had been wet when pressed together.
Hope this helps. Post again if you don't understand anything I said.