Oops! Have belatedly realised a small problem - I usually pin/screw a rail to the wall for the first line of tiles to sit against, rather than start with the rim of the bath itself, which isn't always perfectly straight - but won't this puncture the tanking I've put on the walls?? Am I missing some obvious solution . . .?
I bought a tiletracker, no more batons! Failing that though, you could either take the bath out and start below the bath line, or fix a baton above the bath line, once finished, take off the baton and re-tank the holes you made with some of the tape and primer/tanker left and finish the tiling once it's dried off.
So if you're retiling a bath/shower area, it's not worth taking the bath out to make tiling the area easier? (Even if you're only tiling up to the bath line?)
As stated always fit your bath 1st then tile onto the lip of your bath.
Silicon the bath to the wall then Silicon the tiles that way you are creating a double seal that will never leak water.
Just mark a straight line on the wall and fix your 1st row of tiles. You can use small packers or wedges underneath to raise them to the line. Try not to leave too much of a gap though, 3mm max or your final Silicon seal won't look too great. Good luck.