Add angle indications on keystone

I want to keystone a photo because the left part of the image is distorted (a lighthouse), but the right doesn't have any element (juste the sea).

I can keystone the left part : i can visually align the left vertical line with the lighthouse.

But I can't do it with the right one. And if the 2 keystones doesn't have the same angle, picture is distorted.

So, an indication of the angle of the keystone line would help me. Or maybe a shortuct to make the two lines symetricals