MF is not as strong as people pretend she is, but still does she really need such an AS steroid?
I mean, you're not supposed to autoattack on her lategame anyway. If you get jumped on and your ult gets interrupted, you should KNOW you messed up. My suggestion is swapping the steroid for a flat attack speed buff. Aka, instead of 60, 70, 100% additional AS, just a flat value. What does this bring? She stil gets an advantage by being ahead in lane, but she can't go full hypercarry mode by doublescaling on her AS items. As a result, full ad/pen items become more valuable, and ult placement becomes more mandatory.
What about the actual values? Just get for each rank the value she'd get with just Berserker Greaves. Then if it's too weak buff it.