2F/2F - what's not right?
All the questions in the OP are what I was responding to with the statement.
"If I'm using dilution B, I mix a stock solution 1:7."
1:7 is a stock:water ratio to make working solution, not a concentrate:water ratio to make stock. No matter what dilution you are using in the end, the first step is to make the stock solution by mixing the concentrate 1:3 with water, not 1:7.
"Then, when I process the film, is it correct that I then mix the prepared dilution B 1:31?"
The 1:31 ratio is a concentrate:water ratio, not a stock:water ratio. In other words, it is used when referring to the overall dilution of the syrup for a working solution, not when describing how to use the stock solution to mix the working solution.
Dilution B is described with ratios in one of two main ways: 1) 1:31 concentrate:water ratio, and 2) 1:7 stock:water ratio.
It is the same thought process as using D-76 1:1 or in higher dilutions, except that your HC-110 syrupy concentrate is the equivalent of your D-76 packaged powder. The product as it comes off-the-shelf is mixed to make stock, and the stock is used to mix working solution. The difference is that the off-the-shelf product is a fluid instead of solid particles.
Rereading more carefully, the OP may have explained it right after all, but perhaps a bit clunkily so that it was not totally clear to me that the dilutions were understood.