For example, you can use the 1 in 60 rule in navigation without needing to know why it works (the sine of angles up to 45 degrees approximates to the angle in degrees x 1/60th, if your interested!).
So what happens when you need to use an angle of more than 45 degrees? Like for instance in Radio Nav, when working out the diameter of the cone of confusion at a specified height which is 60 degrees, then 1 in 60 isint quite accurate enough.
Definitely a good idea to get a bit of work in on the Maths. As someone whos Math is described as "awful" by my Atpl instructor I should know. Triginometry was an optional subject when I was in school and my Maths teacher said it was easier to get marks in other subjects like Co-ordinate geometry. So having to do it for the Atpl meant having to stop the class to give a maths class, which made me feel about as popular as a fart in a Space Suit. So triginometry including Pythagorus theorem will be handy to have to avoid this situation. A fundamental knowledge of Algebra is very important, manipulation of formulae is 100% something you will need( I knew how to do that already, thank god). You dont have to go into too much depth with either of these topics just the fundamentals. I would think that the books for intermediate or final secondary school exams would do the job. Anything better will just scare you and be over the top.