growing a mother to pinch or not to pinch

I've just started a mother from seed currently she is about 8" tall latteral shoots aer starting nice so I'm not shure if I should pinch the top to get the lats rocking or if I should just leave her alone. My delema is I need as many little ones as possible before May 24. any sugestions on how to speed it all up oh ya its under a 400 watt hps. thanks for yas help