'Was discussing this topic briefly with my instructor today;
For what it's worth, when carrying out PFLs (and precautionary landings) all three instructors I've flown these exercises with initiated the go-around not at a specific height, but only once certain (well. . confident) that we would make it into the selected field successfully in a real life scenario.
Difficult to say accurately what sort of height this is usually at, as we've often got our home airfield's QNH or the RPS on the altimeter and practice forced landings near rural areas of high ground, but definitely well under 500ft or so.