The best way to pick a harvest date is by watching the resin glands under magnification and also knowing what kind of effect you want.
First comes clear resin glands, those look like glass, all clear is too early.
Next comes cloudy resin glands, those look like plastic. If you harvest when a few are clear but most are cloudy then you usually get a more energetic high.
Then come amber resin glands which are technically past peak THC but it's also when strong stone / couch lock starts.
For meds / couch lock like I like, I usually go for roughly 30% amber and 70% cloudy.
You can pick up a cheap 60X - 100X magnifier off eBay for $7 delivered. Those work fine.
Or you can spend $25 or so and get a fairly good USB magnifier.