From the WhiteWeapons webpage, the speed of a dimensional blade is -20. The automod includes +15% IASpeed, so the final speed is -35. You are welcome to use the
IAS Calculator methods used in previous HU versions. There are probably some more minor details, but I don't have good references for them. If someone provides a link to the details of creating a weapon speed calculator, then I don't mind producing one, but it won't be near the top of my list of things to do.
If you have a link to some D2 documentation that you like, then I'll add a note on my list of thing to do to include or arrange the information similarly. I don't have a way to update the HU Databse linked to along with that calculator link, but I believe it will be brought up to date for HUv1.8.