Unless there is some performance issue that I haven't spotted, it should be sensible to do this the simple and obvious way:

SELECT LoadNumber

FROM LoadTimes

WHERE (@Now >= StartTime AND (@Now < EndTime OR StartTime >= EndTime))

OR

(@Now <= EndTime AND EndTime < StartTime)

I can't see that adding or subtracting 12 hour chunks buys you anything useful, the calculation is just as complex if you do that.