According to the paper I read (link below) 75 cfm is optimal. You make the call to determine if 50 cfm will work for you. Also, while researching various blowers I found that cfm varies from one manufacturer to another. One may say 75 cfm, but really doesn't pull that much. You might want to try to find blowers recommended by someone that has already put a booth together.
http://modeltech.tripod.com/booth1.htm