it seems to me that 65 is the most reasonable breakpoints. Assuming that you will use some ias gloves - loh - and are willing to put an ias jewel in helm or armour - or use ias amulet though I think atma or resistance might be better. Then you only need 30 ias from your weapon.
Therefore, I'd put two 15/40 and an amn. That way you'll hit your breakpoints, get a nice damage boost and get a nice chunk of life leech which is very important. Amazon using fend don't have many means to keep them safe - low defence, no shield and no d/e/a. Constant leech is therefore very useful.