The change in toe as the car changes ride height is the dreaded "bump steer". It can normally be adjusted out to a great extent on a conventional independent front end.
When you grab the two tapes to do the measuring hook them both over the same object, side by side, and pull them out to approximately the same length that you're going to measure. You'd be surprised how often they differ and by how much.
The only other thing I noticed with the setup shown is that the slots are toward the bottom. If you move them to toward the top, when you pull them tight the tops of the angles will clear the bulge at the bottom of the tire and give you a more accurate reading.
I've found most of these thing out the hard way while setting up road race and oval cars. Hopefully it'll save others from going at things the long way round.
Pete