The signal strength you're seeing is not enough information to tell whether you'll have good VoIP roaming
Your experience will be affected by: how large an area, what kind of environment, how many APs, what other WiFi users at the same time, how much interference - and at least 20 more things I haven't thought of.
In my office environment with no interference to speak of and an AP every 75 feet or less, neg65 would work just fine.