the lower buds will never be as done as the top ones. this has as much to do with the physical attributes of a plant as it does with the relative distance to the light source.
you will have to make the call on a grow by grow basis when you want to harvest, you might want to take it when the very top buds finish and have all the lower ones a little pre-me, or you may want to let the colas get a little more amber and focus your timing on the buds a few inches lower. Its totally your call.
grats on your first grow, i hope its smooth sailing from here out.