The best formula I have been given works for reciprocal tracks
Aircraft 1 reports A at 0510 and estimates B at 0550
Aircraft 2 then reports B at 0515 and estimates A at 0558
Add all the minutes 10+50+15+58 = 133
divide by 4 = 33.25
Time of passing is 0533.25
If any time is in a different hour then add 60 to that min
ie if aircraft 2 estimates A at 0610 (slow G/s) then addition is
10+50+15+70 = 145. Divide by 4 give cross time at 36.25
Simple and quick and works for any ground speeds.
Just don't ask me HOW it works!