sjs34 wrote:

My experience was similar.

Edit: I wonder if this one had more subtractions than usual, especially in the top three rows, and if that contributed to the difficulty. My trial and error key came in determining whether 5 went int the 1st square of row 8 or in the 8+ cell next to it. There were only 103 solvers 2 weeks ago because it was a subscriber only puzzle.

In my opinion that's exactly what happens. For instance, we know that in a KS the difficulty level increases if the size of the cages increase, that is, if you have cages with a big number of cells (though being additions, the number of possibilities increase). The subtraction cages operate in a similar way in a Calcudoku. The number of possibilities, in both types of puzzle, is proportional to the difficulty, since they require more analysis and more time to solve the puzzle. So a quick look to the puzzle gives you an initial idea of the difficulty.

The subtraction cages have a bigger number of possibilities (combinations), depending on the "measure" of the cage, and inversely to this "measure", i.e., a 2-cell "5-" cage (let's call it of "measure" 5), in an 8x8, has three possible combinations: [1,6], [2,7] and [3,8], while a 2-cell "1-" cage, in the same puzzle, has seven possible combinations: [1,2], [2,3], [3,4], [4,5], [5,6], [6,7] and [7,8], being, for a 2-cell cage, the number of possibilities equal to the "distance" between the size of the puzzle and the "measure" of the cage (8 - 5 = 3 in the first case and 8 - 1 = 7 in the second case).

A 3 cell L-shape "0-" cage, in an 8x8 (fm 1 to 8) has sixteen possible combinations: [1,1,2], [2,2,4], [3,3,6], [4,4,8], [1,2,3], [1,3,4], [1,4,5], [1,5,6], [1,6,7], [1,7,8], [2,3,5], [2,4,6 ], [2,5,7], [2,6,8], [3,4,7] and [3,5,8] (BTW, a 3-cell L-shape "1-" cage, in the same puzzle, would have twelve possible combinations).

A multiplication cage has in general fewer possibilities, i.e., a 3-cell L-shape "72x" cage, in the same puzzle, has only three possible combinations: [3,3,8], [3,4,6] and [2,6,6].

In the puzzle we are considering we have a total of 11 subtraction cages, mainly concentrated in the 5 leftmost columns, and three of them are 3 cell L-shape "0-" cages. Additionally, the 3-cell in-line "0-" cage in the upper row has "initially" twelve possible combinations.