Redgate Backup - Won't Delete All Files Before Backing Up

I am running backups to a location with limited storage. Because these backups aren't exactly critical, I only need one backup file. I am setting the program to run and delete existing backup files except for the latest "1" and also with "delete prior to start of backup" enabled. However, when I run this, it keeps the last backup and causing space issues on the drive. How can I change the program to delete all backup files before the program runs? Can i change the step in the job within sql and set it to 0b instead of 1b?

Thanks