The Longacre gauge I've used had flats on it that you align parallel with the car side when turning the wheel.
Personally, I think the aftermarket gauges are a waste of money. When I align my car I have a piece of welding rod about a foot long with about 1" at the end bent at a right angle. I put the bend under a lug nut, adding flat washers if it's a taper-seat lug. The rod sticks out in the same direction as the spindle and provides a measuring point for caster. It doesn't need to be perfectly parallel to the spindle nor even level. Once I have the reference rod installed I turn the steering wheel 1 OR 1.5 turns one direction and measure the height of the end of the rod from the floor. Then, after recording the measurement I turn the steering wheel 2 or 3 turns in the opposite direction (1 or 1.5 from centered the other direction) and take another measurement of the height of the end of the rod. The difference in the heights is used along with the horizontal travel of the end of the rod to calculate the caster slope. Using this system you can get as precise and accurate as you'd like and a level surface isn't necessary, though a smooth surface helps.
I have two setups so I can measure both sides of the car at a time and once you have the horizontal travel of the rod you don't need to measure it again. You can also calculate the needed rise/drop from your desired caster spec and you then just need to move the alignment adjusters until you get that amount. It actually goes very fast and I think it's more accurate than reading a bubble in a tube. Best of all, the only measurement tool you need is a ruler or tape measure. For positive caster you want the rearward measurement smaller than the forward measurement, the slope is downward to the rear of the car. Caster=arctan(little#/big#) For example: if your measured height change is 1/2" and the horizontal travel is 12" your caster is tan^-1(0.5/12)=2.4 degrees. With a hor. # of 12" and a desired caster of 3 degrees: tan(3.0)*12"=0.629" or a little over 5/8"
For camber I just use a level with spacers so I reference on the rim. You can quantify this if you want a non-zero camber using the same math as above but the big number is the distance between the points where the spacers meet the rim and the little number is the difference in the spacers or just the clearance between the upper spacer and the rim (for neg camber) with the level vertical.
For turning plates I've used a wad of plastic grocery bags with good results but concrete will chew through them fairly quickly. A pair of thin metal plates greased up work but eventually will get gritty and filthy.
I just got tire of loading up the race car and hauling it to a shop for an alignment, and then wondering what I got for my money.
