Dear Tshirtsteve,

it always depends on the design you use. E.g. white is the color which cracks first. the more white the earlier you will have cracks in your design. But if the areas you have printed are small and detailed the washability will be good.
Of course it is also important to fix the transfer correctly after transferring on the shirt like mentioned in the instructions.
a small raster mask can also be helpful to increase the wash resistance, but like I said before, there is no general setting to have the best result.