Setting "Load balancing" in high-density environment

unknown

Hardware:
ZD1200, firmware 10.2.1.0 build 129

Case:
a lot of people (e.g. >150) in open-space type area. 8-10 access points, channels set manually, OFDM only, BSS Min rate 12 mbps and few other tweaks - nevermind.
Some smartphones are not roaming. I want to help them a little bit.

Question:
How to set "Adjacent radio threshold" to make clients easier to conect to other AP?
Increase the value [dB] or decrease?
What is this value, RSSI?

I know the setting "smart-roam" for WLAN via CLI - sometimes it can help for sticky-client problem, but don't go so far - for now I only want to know about Load balancing.