The UK SMF is set at 3.5NM or less and 600ft or less. The 600ft figure is based around the possible 400ft MODE C tolerance for levels I guess.
Also, although our blips change size outside the protected range of our RADAR's, SMF does not alter it's tolerance as it is not quite that clever.
I wouldn't recommend testing it out though, just refer to the owners manual if you are not too sure.
