30mm is a ball park figure that most sports bikes want to be set at .. you can fine tune it after .. but it’s a good starting point … First raise the front of the bike so the forks are fully extended .. take a measurement from say the top of the fork to the bottom yoke .. next put all your riding gear on .. then sit on the bike with your feet on the pegs .. you’ll need some help to to hold the bike level .. and take the fork measurement again .. the amount the forks have moved is your sag .. so if it’s moved 50mm you’ll need a 20mm spacer … I’m not sure how thick [diameter?] the spacer needs to be as RB forks are thinner than RC/RD forks . ..
Hope that helps