The answer depends on whether you are looking for a strong hold or light hold, whether your strands are coarse and need more product or fine and easily weighed down. It also depends on the product you are using. Some products will require a larger amount to get the look you personally want (light with lots of movement and a little frizz is ok or stronger hold with a little more weight). For some products, a little goes a long way.Part of the trial and error needed to figure this out can start with the recommended amount in the product instructions. Otherwise you could start with a nickel size amount or less for each half of your hair and see how that feels, see how it spreads through the hair (spreadability of the product you use also affects the amount). Ultimately, you won't be able to tell whether that amount works for you until your hair is dry and maybe until after you see how your hair acts the rest of the day. If you don't mind experimenting, you could try different amounts and or different products on each half of your hair. Maybe on a day that you do go outside (to see how it stands up to the elements) but when you are going anywhere important. Even if there is a good rule of thumb, it may not work for you, the products you use, or the look you are going for at all. So, experimenting/trial and error is key.Sorry it's not a clean cut answer but I hope that helps!