Just like the photo - it’s where your skin creases when you bring your leg up. Stand up, raise your leg so that your thigh is parallel to the ground. The crease is where the 90 degree angle between your leg and torso is formed.
You should judge geared powerlifting. Seems most of the judges don’t either.
Maybe I just have no perception of human measurements, but this seems very short for your height, no?
I don’t know. I have no frame of reference until other people share theirs… And I don’t think it’s short. My legs are plenty long.
Now I need to measure my legs. Will report back tonight.
I think yours must be short. Mine were 16 1/2 but only 19 1/4. 
Mine seem to be about 16” in length. I’m 6’ and mostly torso.
We might have some measuring variation problem. Too much Six Sigma trapped in my brain.
If I measure sitting from the crease at my hip to the top of the knee I get 18". At 19 years old I measured 6’ 1/8" (A group of us got together after numerous people said they were 6’ tall, yet some looked shorter. To settle the disagreement we attempted to take accurate measurements.) Today I am, maybe, 5’10 1/2" now at 72 years old.
I have always considered myself long legged and short in the torso. Also, I measure seated 23" from the floor to the top of my knee.
I measured to the top of my kneecap - the top when I’m standing. Not the middle of it. And now you have me wanting to measure my tibia…
Edit: Yep, I’m doing this
25" from floor to the top of my knee. And if I measure from my hip crease to the edge of my kneecap while seated then it’s about 20".
That makes more sense since you are super sized.
Using my measuring method your thigh length would have been 19". I measured to the top of my knee cap.
Aren’t your calves meant to be wearing masks?
I really need to get a measuring tape. I’m missing out on all the fun
Some of us need to find explanations for our appearance. My go to is that I’m stretched out.
That’s why I snapped the photo. The measurements also change when I’m standing. From hip crease to floor was less than the sum of those other two measurements.
First of all, all yous giants can go have a seat somewhere else. I’m barely at 13 inches from hip crease to top of knee. My torso and femurs are roughly the same length though. Give or take a bit less than half an inch for my torso.
I call those good squat genetics!
I can trade my 17" or 18" femur depending how you measure. I only stand 5’8"
Squat suck.