As you stand in front of the engine bay and stare down on the crank pulley, the timing marks move from left to right (in LHD, that's from passenger side to driver side). So the first mark you see is 30 BTDC, then 5 deg decrements to 0 (TDC). (Note - 15 BTDC is the same mark, regardless of which end of the marks you start from)
I have observed that some timing lights "double" the reading, so to get 15 BTDC actual, you need to set it up at 30 BTDC.