Flexbond should be okay but I prefer 1/2" Wonderboard over 1/2" Hardibacker for wall applications. I assume that you are limited by the selection of thinsets at Home Despot? Custom's "Porcelain" thinset is going to be a better modified thinset and give you a stronger bond for your porcelain tile, though.
Best of luck,
Capua Custom Tile & Stone; Seattle, Washington
Member- National Tile Contractors Association
Card-carrying Schluter Systems installer
Certified WEDI Board installer
NuHeat radiant heating system "Qualified Installer"
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