Fill the gap between the tile and the tub with caulk.
Tile over tub flange.
Start your first row just about less than a full tile above the tub edge by using a level straight edge tacked to the wall.
They are also easy to clean and can be found in numerous styles colors and sizes.
The best approach when installing cement board or other tile backer around a tub is to keep the cement board out of the lip area.
That flange needs to be covered or it will look pretty strange.
After you finish the wall install the bottom row.
Most three sided tubs will have a tile flange raising upwards from those three sides.
That means building a small wall down on that end of the tub that comes up and wraps over the tile flange with tile.
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Maybe cap it off with similar material as your countertop.
Divide the dimension in half and make a mark there.
Above the lip of the tub.
Tile on tile or as i say it.
Apply it to the wall with the bottom edge about 1 4 in.
Then apply a generous amount of thin set to the bottom bathtub tile and position it 1 8 in.
1 8 from the front edge of the densshield by the way the tub manufacturer oceania in quebec is adamant about no mechanical fasteners in or above the tile flange only silicone to the studs painters tape the tub then fill gap and onto the densshield with kerdi fix then apply kerdi tape and smooth out after drying redgard as much as possible.
To tile around a tub or shower start by measuring the height of the back wall from the lip of the tub to the top of the area to be tiled.
Tile are a very common way of finishing the walls in remodeled and new baths as they are impervious to water and highly stain resistant.
I plan to add to 3 8 spacers on the studs at flange area so as to bring the front of the tile flange just back approx.
Check if you need to adjust this mark to avoid laying slivers of tile at the top and bottom.
You may have to cut the bottom row which is normal but it will give you a custom finish all the way down to the tub edge.