Hey!
I think that you shouldn't focus too much on the scaling of the subjects. The scaling is just there to balance out the different levels of competitiveness in the subject cohorts.
So, I'd say choosing the subject you want to do depends on how much you enjoy each of them and how well you do in them. You've said that your grades are higher for Legal studies, so which one do you enjoy the most?
Also, this is my personal experience but I've found that Specialist Maths took up a lot of time. I dedicated more time and effort to this subject compared to other subjects. So, I think it's worth considering the differing time commitments to Legal studies and Specialist. For e.g, if the other subject you are doing are difficult and/or require a lot of time and effort or you have other extracurricular/work commitments, then maybe taking on Specialist wouldn't be a good idea. But then again, if you really enjoy it, you could still take it.
So, when it comes down to making a decision, it's a good idea to consider all these different factors and then see which one you are leaning towards.
Hope this helps.