In the good old days, I trained CAA/NATS in 1980's it was always 90 deg line mid point runway and arrival sector and departing sector! I.e geographic separation. This exists fine until a go-around then we earn our ATC pay! Obviously as weather conditions go down then possible go-around needs to be factored in and this seems to change with the unit.
Plus units with multiple runways have their own separation standards. My last unit, I retired last week, a non UK unit with two staggered vortex dependant runways we operated 'segregated' in VMC i.e. normal gaps down to 3nm or less with visual separation. Then in IMC they became 'dependant' and min distance between arrivals became 5nm more normally 4nm. In LVO's this went to 8-10nm.