If you'll allow me, a little story (belatedly) about the tolerances of SMF. (As AREA52 said, it's probably not a good idea to test it).
Shortly after the introduction of SMF in TMA (and shortly after getting my first licence, so I was easily encouraged by all my 'older and better' colleagues), I had an inbound descending to 6A, about 12nm out from final turn, with outbound from nearby airport climbing to 5A.
OK so far. Inbound screams for descent... I asked him if he was visual with the traffic (about 6 miles at this stage, 2 o'clock). Minimum looked like being about 2 miles (min in TMA 3 miles). He reported traffic in sight, and could descend with traffic in sight.
Then I had a brainwave (remembering SMF). I told him to turn off transponder 'because it sets off alarms' as I put it on the freq!
He did, descended, switched the transponder back on, and went away to approach, a happy bunny.
Five mins later the supervisor walks up...
" Who just got 1.5 miles between these two???"
Accompanied by much laughter from my dearest colleagues, I held my hand up.
Bloody thing extrapolates between last known plots, doesn't it!!!
(if any one tells you to 'switch off transponder' if they're getting tight, you now know where it comes from. B**tards never forgot it.)