The outside forearms are typically the darkest areas of a person's body, owing to it having the highest sunlight exposure. Because they are continuously exposed to the elements, consider structuring your designs around that concept. Avoid putting the most detailed elements of your composition in these outer regions, allowing for the most robust possible value range.
Also, consider that the forearms are what the audience might see first on ANY client. Utilizing a simpler, more robust value range in these areas will not only garner more longevity out of the tattoo, but also increase readability from greater distances.
Remember to keep the body parts and forms in mind while developing your tattoo designs. Planning around these different aspects might be a lifesaver for your design. Allowing a wider range of value in your lightest, most detailed regions, as well as bigger shapes with a lower value range where the sun may touch or the eye might catch.