When I was growing up, basketball was big in my neighborhood. Everyone wanted to be able to dunk on a regulation 10-foot high basket and, thus, everyone focused on improving their vertical jump. The progression usually went a little something like this: touch the rim, grab the rim, hang on the rim, dunk with a volleyball and, finally, dunk with a basketball!
Scaling the back squat for beginner-level athletes generally entails sticking to lighter loads (even bodyweight only to start) while learning proper technique. Goblet squats with a kettlebell or dumbbell can be used to practice form, but keep in mind that goblets are an anterior (front-loaded) variation and won’t directly mimic the mechanics of the back squat.
A second, more efficient and correct method is to use an infrared laser placed at ground level. When an athlete jumps and breaks the plane of the laser with his/her hand, the height at which this occurs is measured. Devices based on United States Patent 5031903, "A vertical jump testing device comprising a plurality of vertically arranged measuring elements each pivotally mounted..." are also common. These devices are used at the highest levels of collegiate and professional performance testing. They are composed of several (roughly 70) 14-inch prongs placed 0.5 inches apart vertically. An athlete will then leap vertically (no running start or step) and make contact with the retractable prongs to mark their leaping ability. This device is used each year at the NFL scouting combine.