that's a good start. Its a difficult perspective, but you are doing well, so far.
I use a pencil to help me determine the size ratios of things to each other. Sometimes we THINK something is bigger than something else, so we have trouble ptting them into perspective correctly. But if you use a pencil, up against the screen, and measure, like this:
The distance between the rider's elbows is the same as . . . . And use the pencil and find soemthing on there that is the same, and then draw them the same.
Or, the distance between the pupils of the eyes (on close ups) is twice the distance from the pupil midline to the nose.
You compare one known thing to another, and thus you keep the relations accurate.