The most important thing about making your own race track is keeping it as fair as possible. You don't want any outstanding advantage in one lane over another...but that can be difficult.
Given the nature of plastics and other materials, it's likely that your track will not stay the same between races. No matter how hard you try, you'll setup your track differently each time. So calibrating your track before you run any "official" race is your best bet.
The nice thing is, you don't need any fancy tools to calibrate your track. I mean, if you got 'em, use 'em; but I don't have any such accessories so I use what I got - my eyes.
Use matching cars
When I need to calibrate my track, I go and buy 2 cars of the same model. They go right from package to the track. If they're the same color, I use a Sharpie or sticker to denote the difference. While every car will be different (even same-model cars), in theory, they should be close and race similarly.
If you have a timer on your finish line, you don't even need your eyes...but for the rest of us, I just denote how much of a lead the winning car has over the other. I use a 1-3 scale: 1 meaning the finish was super close, 3 meaning the finish was a blow-out. Given this isn't scientific, it seems to work just fine.
I then run each pairing 5 times per lane and jot it down on paper.
If your track is fair, you should see the same results in both sets. Even in the case of one car blowing out the other, as long as the losing car gets blow out on each lane, that should be okay...just means that car is slow right out of the box.
If you see a lot of variance within your result sets, then you need to figure out how to fix your track and reduce that variance. So what should you do?
Adjusting your track
First, check the obvious things...
Make sure your track is flat and straight. If your track has waves or bumps in it, use Velcro to keep it down, or just swap that segment out for another. You can use popsicle sticks to help keep your track straight too.
Try keep your segment joints as even and smooth as possible. The more you race, the more the segment joints will expand, bend and flair out, which can cause cars to catch. Toss out overly worn track and slide in a new piece.
After that, I've found that most unfairness happens with the transition where the hill turns into the straight. Unfortunately, this one is sometimes tough to remedy.
Make sure your track at the bend is the same for both tracks. Make sure they have the same amount of give (or no give). And make sure the downhill stretch is straight and lined up with the transition.
Try moving your starting gate left/right and that will align the downhill track differently, and hopefully fix any issues. You will probably need to do this to account for any variance between transition bends. It's a back-and-fourth until you feel it's giving your consistent results.
Personally, I find track calibrating incredibly frustrating. I have to constantly remind myself that this isn't a scientific endeavor that has great bearing on the future of mankind. No, it's just a plastic race track on which I race toy cars for the amusement of others. After saying that to myself a few times, I usually feel better and don't sweat the details.
As long as your track doesn't show overly obvious signs of unfairness, it'll all be good. Just remember you'll never get things perfect. You'll have problems and that's okay...just have fun doing it and have fun racing. In the end, we're just racing Hot Wheels here.