R = 1.4 (but things have changed drastically since then), I suggest if your building a hearth you consult with the local building inspector before having him come over to sign off on it (saves a lot of aggravation down the road... he will tell you what he likes to see), the clearances can then be given to him from the manual you have in hand (or they can found in the coal burners almanac here Copy of manual for Crane 44
Off the record... the Crane 44 is about the safest coal stove I've ever seen, I have run them full bore to the point I could almost see through the side wall while they sat on factory fir flooring
(I don't recommend you do this but just saying).
also Michael is correct... you cant buy asbestos millboard anymore (and you would not want to anyways). You can buy safer non-asbestos products now... but the answer to your main question is 1.4 was the R value in that example.