A couple of 1 1/2" x 1 1/2" x 36" pieces of angle aluminum - or steel - set on a couple of short 2 x 4 blocks that are set on edge so as to get the angle aluminum pieces up off the floor away from the bulge at the bottom of the tire work well.

Some guys drill the angles and bungee em in place.
Measure the angles transversely, equidistant from the center of the wheel hub fore & aft.
Just barely ahead of the tire's extreme fore and aft dimension horizontally works well.

I run 1/16" - 3/32" toe-in on my street driven 32 roadster.

About 6 degrees of caster makes for good handling and easy steering.
Much more than 6 degrees makes the low speed steering a little harder as far as input effort goes.
Not a problem on a comp car.

I did some research on dry lakes/Bonneville roadsters a couple years back when I started building a 31 roadster on a 32 frame for the lakes.
Some reported that 12 degrees was the way to go and others said 8 - 10 degrees.
I built the chassis for 8 degrees with room for more if necessary.
It had the capability of going back to 4 degrees if desired and that turned out handy when I decided to put the roadster on the street with 6 degrees instead of running it at the lakes or dragstrip.

Many - if not most - garage floors are tilted about 1 degree toward the door so water will run out.
Since you'll be measuring caster against a flat surface and not the flat area of the frame rails you may want to account for that by doing a 180 with the car and shooting the caster again.

Fwiw - I use one of the circular angle protractors, mine from Sears and about #10. a few years back.
They work well.

If you want real precise, get one of the shorter digital levels and make a simple aluminum block so you can adapt the digitial level to the kingpin boss.

The digital levels measure to seconds of arc.
I have a 4' model originally purchased for a plane I was building and it turned out very handy for setting up the 3 frame rails.

The shorter levels can be rubber banded to a straight piece of aluminum channel the length you desire.
That's not a bad way to go, the 4' levels are to an extent useless for most car building stuff.

Then again, there's always the hacksaw....