Nut Replacement

Hi there

I just picked up a sq neck hounddog dobro el cheapo on ebay and i'm going to set it up with some new parts, most of which i'm fine with.  However it has a tusk nut so want to replace it with a bone nut.  I've never done this before, but i have read the article on the stew mac site about removing it so i'm half way there.  The thing i'm wondering about is the angle from the bass to the top string.  I notice the nut slopes up to the 1st string (i know why) and wanted to ask what that taper should be?  I don't want to guess, but if there is a standard measurement somebody could give me that would be great.

Also, what is the sound difference between tusk and bone?

Many Thanks

Ben