Order a 60x jewelers loop off amazon for like 9 bucks very useful when checking your trichs (80 milky 20amber) it would give you a WAY BETTER way of figuring harvest dates
If you can't cop out 9 bucks, you can take a wild guess by watching the little hairs pull back in the bud and turn orange red type color and by checking the estimated flowering time according to the strain you are growing
Have fun! Let me know how it turns out