I’m not assuming anything, it is more a calculation based on the experience on previous users that have studied German and shared their experience. And they were advanced users that I respected. It is a bunch of old discussions that I have no idea where they are, also because we have changed forum. But at that time I was taking notes and I was crunching numbers to get a reference and goal to have.
As I said, I’m a bit conservative. Keep in mind that there are people that have number like 130k known words. We are not talking about lings (yellow words), but real converted known words. Staying at 65k known words it is a good figure, probably 55k known words would do.
I have no idea how you can not find new words with only 11k known words. I wish I had that experience but I haven’t. I have 45k known words in German and I consider myself intermediate. But I don’t read the same stuff, and I often change topics.
IF you only take the official Lingq Threshold published by Lingq in this link, you will see that they consider Advanced 2 with 42K known words (more or less).
Keep in mind that Advanced 2 could or could not give a C1 level. Of course it depends on the material you are reading. But that’s the minimum. If you are lucky to nail the most common words, maybe with a minimum of 42K you are advanced. But as I said, I prefer to have a higher goal to avoid demotivation, and 65k known words was the goal that I had written a couple of years ago, or maybe more.