What amount of cell overlap ensures smooth roaming for wireless endpoints?

answer choices units off by a factor of 100

“Also, consider providing a 15~20% cell overlapping to ensure there is smooth handoff of the video application from one access point to another when the clients are roaming” – http://www.cisco.com/c/en/us/support/docs/wireless/5500-series-wireless-controllers/112889-cuwns-vidstrm-guide-00.html