Multiply the opacity and fill opacity fractions, and you get the final opacity value. In your case this would be 0.65*0.5 = 0.325. So you're right that halving the opacity in this case works.

However, there are certain pitfalls that make a straightforward conversion from photoshop to CSS difficult. Fill opacity in PS will not affect drop shadows and such, but CSS opacity will also affect any analogous box-shadows you apply.