live rock & tufa

[vic622]

I'm in the process of setting up my first SW tank, a 45gal with a 29gal sump/refugium.

I'm going for light stocking, Bengaii Cardinals (2,3 or 4), and a goby-shrimp pair - after cycling.
I'm also thinking of getting a detritivore kit -
http://www.inlandaquatics.com/prod/prod_detrit.html
to kick-start the sand bed (3"-4") & red mangroves for the refugium.

With a deep sand bed and a refugium, how much live rock will I need and how much of it can I replace with tufa?

Vic