Proportion grid
Load a reference or a photo of your drawing, lay a grid, and drag the red lines to measure. Your image stays on this device.
Drag the two red lines, one horizontal and one vertical, to any points you want to compare. The readout shows their position as a share of the image.
How to use it
Photograph your drawing square on, at arm's length, then load it. Set the grid to match your paper, for example 4 squares across for a quick check. Compare where the eye line, horizon or main edges fall against the rules listed on the right. These are rules of thumb for a typical case and people vary, so treat a gap as a prompt to look again, not as proof of an error.
Related faults: all measuring mistakes, and for tone rather than shape try the value squint.