Hey irish,
From all the info I have gathered, I understand that if you harvest when less than 50 % of trichs are amber then you will get the uppidy, head like high... Since you say you want something "hard hitting" I think you may be looking for more of a "STONED" type of high, that type of high is achieved when more like 60%-80% of trichs have turned amber... As I said before, this is from the info I have gathered, not personal experience....
You are more or less right, but by 60% to 80% amber he will likely be losing a lot of THC to oxidation, but regardless, he said; "The strain is unknown.." and you have to factor that into any advice given.
The whole harvesting when clear or cloudy/milky or light amber of dark amber thing is only fine tuning the strain type, what the genetics hold and will give, at the end of a grow. He's 10 weeks into flower and just now starting to see a little amber. That pretty much tells you there is it is likely that there is at least a tad bit more sativa in his mystery strain than indica. Maybe not much, but at least a bit more. How much is impossible to tell, but it is still likely.
But then attempting to fine tune a harvest you cannot make a really big difference, only a slight one. Say it were a full indoca and he wanted a clear soaring head high of a sativa. Even if he harvested when some trichomes were cloudy and many were clear it would not turn the indica genetic mix for cannabinoids into a sativa mix. And if he had a full sativa and wanted couch-lock no matter how much amber he waited for he would not turn it into a heavy stone indica buzz.
As I said you can only fine tune and regardless of what someone wants it is not rational to go beyond peak potency time for harvesting trying to turn something into something it can and will never be.
For many years on many boards I have read messages where someone purchases a 'flavor of the month strain' and then planned to, and later attempted to turn it into something it was not and never would be by harvesting earlier or later. They pick the strain because it was the one most people were talking about, because it was the 'flavor of the month' strain but not because of it's genetics being what would give them the high or stone they best and most like. Still, come harvest time they would harvest an indica early going for a sativa buzz or harvest a sativa late going for an indica stone. In the process they ended up not making the most out of the genetics they were growing. They would play the part of a grower alchemist and harvest too early or too late trying to transform genetics they should never have picked in the first place, given their likes and dislikes, and attempted to overcome thousands and thousands of years of evolution and genetic coding in hope of giving them what they wanted.
That is why I will so often get on my soapbox and preach forget about the 'flavor of the month strain' that so many people are talking about and pick genetics that will give you what you want the most, even if it is a 10 or 15 or 20 year old strain instead of the new kid on the block that has become the 'flavor of the month.' strain.
Being a mystery strain, and highly likely a cross, and with the first signs of amber showing up at week 10 it might be more like a 60/40 sativa/indica and he could get pretty much of what he wants, but I would not go past the point where any trichomes became a darker amber. Once past a light amber THC is oxidizing, degrading, being lost at a faster rate than it is being replenished and it is turning into increased levels of CBN. At that point potency is on a downward slide and it will not reverse itself, it will only get worse and worse, or better said less and less or lower and lower.
My advice is make his mystery genetics the best they can be, regardless of that possibly not being what is wanted. Don't detract from the best it can be trying to play alchemist and trying to transform something into something else it cannot be transformed into.
If he wants a heavy hitting knock him on his butt and leave him there indica the best thing he can do is harvest his mystery weed when it will be it's most potent and then for his next run purchase something like Mr. Nice Seeds Medicine Man (the original White Rhino) and grow it right and harvest it when its at it's peak potency level and then have what he wants.
Mystery genetics are difficult to give sound accurate advice on because they are just that, mystery genetics. You do not know there specific needs. You do not know a general flowering time. You do not know their genetic makeup that would tell you at what trichome color it will give you the most potency it can possibly give.
Those are some of the reasons why I hate to see people growing from bagseed or using seeds friends gave them that they either did not know what they were giving or just didn't tell the person, or even the mixed seeds packs that either just say they are some combination of indica/sativa or sativa/indica mix (meaning the first name is predominant in the cross). It leaves you in the dark about things that are important to know and if someone accepts general information as being specific to what they are growing they can end up losing out, or at least losing some, in the end.