In order to believe in free will and also to believe in maths, it is necessary to also believe (for example) in the free will or completely random behaviour of individual cells, or maybe that the third time you combined red and yellow paint you might get black. Or C14/H18/N2/O5 in a vacuum would create a tomato flavoured yoghurt, just once.

It doesn't seem reasonable to say that things are "a bit" random, but also “a bit" predetermined

It does seem reasonable to say that the variables aren't yet known

In objective reality therefore, surely everything that will happen is pre-determined.