Toe-alignment with a string only works if front and rear wheel-track is exactly the same and equally sized wheels and tires are used.

So measure track-width front and rear seperatly. If they differ, don't use a string because you'll end up with a tire-scrubbing toe-in or toe-out.

Every smartphone these days has apps that can measure angles in degrees, so setting camber is easier than ever.
Some apps can also help in setting caster.
You'll just have to fab a setup where you can securely mount the phone to the wheel properly so you can adjust the cam-bolts.