looking great in there Buddy. If your having to go by the general look of the pistils ( hairs) then when you get to about 70% browning/orange of the whole plants hairs. It was the only way we could judge it years ago. up close with your loupe , they will look like a grain of sugar and wont allow light through, this is prior tothe sweet spot. Even with your 40x , you will easily be able to notice the amber portions as they stand out when they turn. Well done for it just going by the weeks and thinking with more focus on harvesting correctly. There are phone apps that offer a microscope, free and not the best but do work good for looking . Good luck mate. keep up the great work