‘Les Jardins de Marqueyssac’, in the Dordogne
Tradition has it that box topiary should only be clipped after Derby Day (3rd June 2017). With all the other demands of a June garden, I rarely get around to clipping mine before July. Don’t leave it much beyond this date otherwise that sharp line that you are after will be buried under a surprising amount of foliage.
When clipping my buxus balls, I always brush them with my hand afterwards. This helps to more clearly see any leaves that I have missed. I like to give my buxus a good feed and water after clipping.