When analyzing a client’s eyes you’ll first want to examine lash health. Length of the natural lashes will be the first factor. Typically fare skinned blondes have thinner lash thickness and will break faster than the average where brunettes typically have a thicker lash and African American’s typically have a thicker lash often naturally curly (may need more than one attachment points) and often Asian lashes slope down. This being said I have often seen the opposite but the point of brining this up is that you need to factor in not only lash length when determining lash extension length but also what the natural lash can hold. Don’t judge based on their natural hair color and width but put a safe lash extension on and see how it wears. After a few fills you’ll be able to tell if you can safely go longer or thicker. 
Remember the keys .07 4 to 1 ratio .05 8 to 1 and .03 16 to 1 use these to start but remember your judgment is best. Always go shorter and lighter if possible you’ll be able to have your client wear them longer with less risk of damage.

Happy Lashing,