* CORRECT DISTANCE FROM SUBJECT TO THE CHROMA KEY BACKGROUND MATERIAL *
The ideal distance from the back of your subject to the chroma key background screen is (10 ft.) ten feet. (8 ft.) Eight feet is also ok, it's when you start to close that gap from there is when the "spill factor" potential creeps in.
If you have your own studio then you can spread out your equipment as you like and get the best results. On location event photographers are not as likely to be granted large display areas for you to setup your on site photography chroma key studio/booth. Just know that the closer your subject to the chroma key background screen, the more likely the green or blue screen color will "spill" onto your subjects hair, body and props used.
Doing full length portrait chroma key photography has more challenges than when you are just shooting head shots, waist up and from the knees up. When you are shooting full length, you will have a chroma key screen under your subject as they stand on it. Now you have an additional automatic spill factor built into your shooting/lighting formula. You may still need to do some additional post editing (removing spill and or doing a garbage mask) in a photo editing software program like Photoshop when shooting a subject full length.
Always keep in mind the type of props or material (clothing & accessories) people wear at a photo shoot, whether in studio or on locations. Leather in general (shoes, belts) are shiny or reflective and pickup the green or blue screen colors. So does jewelry or chrome on wheels, swords or painted surfaces on cars. These types of materials are like mirrors, reflecting objects and light around them.
For people: If you are out of extra lights, that's ok, try using reflector screens to bounce your lighting onto the lower areas of the subject. Or just go to your local art supply store and purchase some white foam core. For optional effects you can buy gold (metallic / reflective) matte paper stock and glue it onto the foam core for additional strengthening and use it for a colored gold reflective screen. Bounce this additional reflector light onto your subject a little from behind them, not straight from their side. This will also help with any "spill" coming from your background chroma key screen, that would effect the lower portion of your subject (feet area, etc.).