The crumble topping should be about 2cm thick. Firstly, rub the flour with the butter to get rough, uneven crumbles. Now add in the rest of the ingredients, including the sugar and spices. Cover the fruit layer with the crumble topping; makes sure that it covers it thoroughly and is at least 2cm thick. This is a sufficient amount of thickness for your Crumble, without the crumble topping or the fruit layer overpowering each other.