Can you confirm the race start line is drawn from the starting mark, to the flagpole on the starting boat?
It should always be described in the Sailing Instructions. But as a general rule, that is correct.
Sailing instructions will usually define the line but may not be the mast.
There may be a crowding mark at the boat end of the line to save the paintwork on the start boat.
As above, always read the Sailing Instructions WELL before each event. That said, I was at one event where the RO put the Orange flag up the forestay of the trailer yacht start boat and then sat in the cockpit. He didn't like it too much when I corrected him and suggested putting the flag on a boathook and taping that to the stanchion , that way he could sit in the cockpit and look straight down the line.
As a NRO accredited by AS/YA/AYF, I also teach the art of Race management.