Thanks for all the replies guys!
It's interesting to hear how other people are nailing that spacing and it seems like I'm not the only one who can't explain it more than "eyeballing the relative distances and speeds".
In regards to the loss of wake turbulence separation I've heard of busy units where apparently it has become as standard practice that you, up to a certain point, only warn an aircraft of wake turbulence (let's say when the loss of separation equals to less than 0.5 nm) and ask them if they want to continue. However, if the loss of separation will be greater than this the aircraft has to be taken around for another approach. I would imagine that a procedure like this would have to be approved by the "appropriate authority" and described in the MATS part 2. Has anyone else heard of this?