I'm a bit behind on this -- can someone point me to some documentation concerning a formula for the calculations?
Someone told me the base speed of the offhand doesn't matter anymore. Is this true?

It does not matter anymore.

You just need 42ias with a -30wsm base.

AWSM - average weapon speed modifier, is no longer a thing anymore.

Meaning, you can use a GT in main hand a war fist in offhand, but still only need 42ias on main hand (or additional ias from sources other than offhand)

Thanks! This definitely cleared things up.