Micore 300 & hearth question.....

So I'm getting ready to build my hearth for my new Englander 30-NC which requires a 1.5 R-value....

I managed to locate some 3/4" micore 300 but cant' locate the R-value. The 1/2" stuff is listed as a 1.1 R-value, so the 3/4" stuff should EASILY get me to the 1.5 R-value, right?

My plan is frame the bottom with 1x3's, then 3/4 plywood, then 3/4 Micore 300, then 1/2" wonderboard and top it off with either Granite tile, or 1-2" granite slab --- (I'm hoping my granite guy will hook me up). Does this sound okay? And to what should I use to secure the Micore to the plywood? Obviously I'll use mastic & screws to secure the wonderboard to the micore/plywood...

Thanks!

Actually -- I misspoke.... I'm going to use Hardibackerboard not wonderboard.....

I think the Micore alone will get me the r-value I need, right? the Hardi-backer is just there to have something solid to attach the granite slab or tiles to....