One "measure" may be physical disc sales.
Here are the
combined DVD/Bluray sales for the respective movies since being bought by Disney:
The Force Awakens - 7,434,520 units in 2016
Rogue One - 3,985,851 units in 2016
The Last Jedi - 4,056,507 units in 2018
Solo - 2,185,422 units in 2018
This obviously doesn't account for digital sales. But as we see The Last Jedi underperformed compared to The Force Awakens but that was going to be a given. What's interesting is how close Rogue One is to The Last Jedi's sales. And the Solo obviously suffered the most (that movie I saw the most comments online saying they would wait to watch it on Netflix)
If I do some totally baseless math and say that The Force Awakens DVD/Bluray sales represents 100% of the Star Wars fan base I can that divide the Last Jedi sales by Force Awakens sales and see that TLJ sold only 54% of the total TFA sales.*
And then I can throw out there that 46% of the Star Wars fans who bought TFA didn't buy TLJ and just say that's the percentage of the fan base that hates the movie.
*By the same token TLJ's domestic box office was 66% of TFA's. Also every middle Star Wars film has sold less then the first movie in the trilogy.