i am assuming you are trying to make the job easy and run a full tile around the bath top!!!!
if the bath is 100% level around the rim ok but if it wanders out by say 5mm
you have to remove a greater amount as you go up to 5mm or try to add progesivly thicker slithers up to 5mm
ok you think follow the bath edge !!!! well no the part/full tiles at the corners will be on the slope
best aim for 50 to 75 percent tile cut on the bath then you wont notice any deviance