This page should help here:
The FileMaker Bible by Ray Cologon has a section on Order of Operations which some additional information if you have a copy of that.
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Arithmetic operators use the standard order you probably learned in primary school: BOMDAS or BODMAS. However that only partly answers your question. FM has a default order of operations within which evaluation of arithmetic operators fits. I commend to you the relevant section in Ray Cologon's book as it is far clearer and more detailed than the help menu link in the previous post—although the help menu still is useful.
Thanks Databuzz, that's exactly what I was looking for.
Or just use parens.