Laser Level Grid is an augmented reality (AR) app that makes it easy to hang a picture perfectly. By using the phone in one hand and a pencil in the other, you can eliminate the need to buy traditional bubble levels and even measuring tools. Grids make it easy to adjust the relative distance between images and center them in the available space. Angle finder makes it easy to measure angles instantly.
If the lines look a bit off (not quite equal) when you first open it, just tap the calibrate button and follow the instructions.
Also note, fisheye cameras on phones warp the image at the edges and then the software straightens things out, but it’s often inaccurate. If you’re using the grid feature, use the midlines for the corners and the closest lines to the edges just to check the relative distances.