It's almost (but not quite) impossible to say when ignition occurs in a glow plug engine because of all the variables. However, I have a graph made up by Brett Buck from cylinder pressure readings gathered by Sott Bair in tests done on a Super Tigre 46 running in a 4 stroke at 7500rpm. The graph is actually instantaneous torque at all degrees of crank rotation but it can be interpreted as showing where ignition occurs. The graph shows a sudden rise in torque (pressure) at approximately 25 degrees BTDC.