Yeah, it uses an unconventional style for mysteries that can be hard to judge.
If you know about Brindlewood Bay, it basically works the same way. As a mini-explanation: in “Carved from Brindlewood” mystery games, there is no pre-determiend solution. Instead, there are a bunch of clues.
When you want to “solve” the mystery, you come up with a solution that uses as many clues as it can for evidence. Then you roll 2d6 + # clues used - the mystery’s complexity. Roll 7 or more, and you’re correct (possibly with a complication). Otherwise, back to the drawing board.
So these kinds of mysteries are about gathering as many clues as you can, then weaving a solution that uses as many of those clues as possible.
However, being unfamiliar with the rules, I find it very hard to rate.
If you don’t want to buy two different rulebooks (understandable), I suggest skewing your ranking towards the lower end of your range. It’s the “safer” score, and it’s possible any issues you currently have persist even after learning the rules.