Lol if you want them to stay at 36" then i would do it soon and make sure you keep them around 16' when they enter that stage.They both respond well to topping.You can manhandle those plants with almost no reaction.
Use them at 1/8 what the calc. says..then the next watering try 1/4 and see if they get burnt.Once you come to a tad burning back off and you know what you should be at and what they like.
Hey man I was reading your white widow grows...i'm fixing to start one myself and I was wondering if there was anything that you would only know if you grew a harves before...do you have any advice on nutes an such...Thanks