You can start practising on MS Paint and use the Text Button for brain dump.
Also practise drawing critical paths on MS paint.
I did not get many mathematical formula based questions in my PMP test (~2-3).
You can put comments to each question(there is a feature in Online proctored test) in case you have any thoughts in mind regarding the question.
All the best for your exam.
Whiteboard feature is explained @:
home.pearsonvue.com/op/online-proctored-whiteboard-overview