The biggest problem with a wideband O2 sensor in the collector without exhaust pipes is at idle and low throttle positions. I think you will find that 7" from the outlet of the open collector will be useless at low/idle throttle positions. Around 15" should be fine, the more the better.

Mine is around 10" from the outlet, and it works fine at WOT going down the track, and also footbraking at 2500 rpm at the starting line. I use collector extensions or my bullet mufflers for idle and low throttle numbers.

No magic number, just rough guidance.