i'm not site survey expert but i will share my thoughts :
1) Why cell overlap should be 20% in voice deployment , why not 30% or 10% ?
30% could be considered very high which may resulting in interference with adjacent neighbor channels ,,
2) Once we get site survey report with heat map , how to verify the cell overlap percentage ?
this is smth i would like to know , but i would say that if i get -67 at the AP edge cells and farther the client will find an AP with RSSI > -67 , then i would say this is a 20% channel overlapping or so .
The recommended AP cell overlap for VoWLAN deployments at 2.4GHz, is 20 percent. The purpose of the 20 percent overlap is to ensure that a VoWLAN handset can detect and connect to alternative APs, when it is close to the cell boundary. This should allow a VoWLAN client to change AP associations with a minimum of interruption to a call, by minimizing the amount of data rate shifting and retransmission at a cell boundary for a given VoWLAN client.