I (Kaijo) first performed a successful brew with a crafts.culinary.brewing bonus of 230. Note that I first tried it with a bonus of 217, which failed. I then wore a bronze helm, which bumped up my bonus to 230, which DID work. In order to find the exact bonus, I will need to level up brewing a bit more, checking the bonus each time. I personally believe it's at a bonus of 225, but it very well could be 230 exactly. Just note that a bonus of 230 DOES work, and the actual bonus is between 218 and 230. In addition, I had not leveled up distilling much beyond that required for vodka (it's at a bonus of 153), and it still worked, so it looks like distilling isn't required as much.
A bit after this, I leveled up crafts.culinary.brewing until the bonus was 218, and tried again. It worked, and since it doesn't work with 217 but does work with 218, it seems 218 is THE bonus required. BTW, I TM'd the skill when I tested it, so it's in TM range as well I guess (bonus didn't increase when it TM'd, so the result shouldn't have been affected by that).
However, there was about ~1 pint of water mixed in with ~2 pints of scumble when I was done, when with vodka it was pure when it came out. Perhaps distilling would reduce the amount of water in the final product? Though that's just speculation, and certainly needs a bit more research.
In addition, the ratio is definitely 1:1 for scumble right now (I pour in 1 bottle full of apple juice and marsh water into the vat and it works), but I remember the ratios actually changing slightly as my skills leveled up, before I had the skills to actually ferment the scumble. I suspect that your character thinks they get the ratio right at lower bonuses (thus starting the fermentation process), but they actually do not (thus resulting in nothing happening). Then again, this could just be my faulty memory, as this part of the process happened several months ago.