Script to remove oldest file in a directory

I’ve created a script in order to manage the /var/spool/cups directory. I like saving cups spool files for as long as possible, but cups will only allow one to set the maximum spool files to save. It doesn’t allow one to set a minimum % free disk space. Here is the script. It monitors the % space used in a given path and if the % free space is below the set amount the script will delete the oldest file in the directory until the free space is below the % given. Here’s the code:
SPOOL_DIR="/var/spool/cups" #location of cups spool
MAX_USAGE="80" #maximum % disk space used 

while [[ $(df "$SPOOL_DIR" | tail -n1 | awk '{print $5}' | cut -d"%" -f1) -gt "$MAX_USAGE" ]] ; do
        ls -t $(find "$SPOOL_DIR" -maxdepth 1 -type f) | tail -n1 | xargs rm -v