So, update - I think I can call the calculator a moderate success!
There are some scores it just cannot predict - for example, a Further study score of higher than 45 - and I'm not quite sure how to fix that (yet). However, it CAN accurately predict up to about 42 without any problems - but these problems are only with Further. Some other subjects, like methods, it seems to extremely accurately predict study scores, no matter how high or low they seem to be. This makes sense for a lot of mathsy reasons I'm not going to go into right now, but means I'll likely feel very comfortable potentially walking away from the project this year and let it self-manage.
Firstly, some disclaimers: this calculator only works if the grade distribution for the next year is similar to the grade distribution data it's using (right now, this is the data for 2019). However, this turnaround has shown that the distributions should be similar enough each year that you can still use the old data to numerically predict the new data. This makes sense, because that's how our current users predict study scores already. They use what they know about the subject, and what marks usually correlate to what scores, to estimate what study score you would get with the marks you have. For that reason, I believe that this calculator is about as accurate as any user on the website, with some small exceptions.
Right now, there's only one exception I've managed to find. That one is Further - if your GAs are all higher than 95%, then your score will be underestimated. Between 90% and 95% seems a slight grey-zone, where I'm not too sure. Most other subjects seem to be pretty well predicted, even on the higher end of things. Most importantly, the calculator is really good for scores between 20 and the low-mid 40s for every subject, which is where most people will be scoring.
If I had more time, I would design the calculator to use special error bounds for different subjects, so I could properly release the calculator - but I don't have the time for what was simply a pet project.
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After all of that, simple conclusion - anybody doing year 12 in 2021, you can rest easy that this calculator seems to be an accurate predictor. So, if you really want to predict your study scores throughout the year and to see if you're on track, then you should be fine using this tool.
But also, please read the instructions. The constant e-mails requesting access got really annoying after a while - it might not seem like much on your end, but at one point I got like 10 e-mails all in one day, my phone actually vibrated so hard it yeeted itself off my desk.