Splay angles in veneer plaster (or conventional plaster) ?

An inside corner more than 90o is called a splay angle. Check out this mystic secret.
Recently painted eaves (soffits) are already peeling, indicating a roof leak, perhaps due to a cracked tile.

Splay angles on veneer plaster can be straightened using a 6 foot rod (featheredge). You'll want to straighten the basecoat and the finish coat.

heinous crack is a result of the roof leak

For stucco or conventional plaster basecoats, 2 strings are used. The strings are set close to the center of the angle, not one string right down the center

heinous crack is a result of the roof leak

These strings must be plumb side to side. We used a 6 foot level. The strings don't necessarily need to be be plumb to the wall thickness. If these strings are plumb, your splay angle will be straight every time.

Too bad I don't have more pictures. I hope you can figure out what is going on. The next step is to pack mortar behind the strings forming vertical screeds.

More about screeds here...

heinous crack is a result of the roof leak

Fast forward to finished splay in the brown coat. Splay is dead straight.

The mystic secret is to plumb the strings side to side.