Harvesting early will get you high quantities of THC in comparison to CBD.

CBD is seen as an anti-psychotic and so controls, to an extent, the psychotropic effects of the THC. Read something on this before somewhere. I think it's a scret 'legit' lab in England where they grow weed. The government are involved in it somehow. But the scientists there have found interesting things in the weed. Funny, with all the claims that weed can trigger schizophrenia etc they actually found that weed possesses anti-psychotic properties too. 18% THC is one of the higher end groups of weed I think. I'm guessing that what it says on the packet of seeds.

Just remember that it isn't just about getting THC out of the weed. CBD also plays a role in the kind of high you will get. You might find that you like a high with more CBD in it.

Maybe harvest the plant on 3 separate occasions.

First, when most of the trichomes have gone from clear to milky. This will give you high THC to CBD levels. It will be more of an energetic high. (Early harvest)

Second, when most of the trichomes are milky but you have around 20% amber.

Third, when about 60% of the trichomes are amber. This will give you mare of a heavy high. Stuck to the couch type of high. (Late Harvest)

From these 3 types of smoke you can decide which you prefer and make note for the next grow.

Keep me posted on your results.