So much work... if the chipboard is fairly flat why can't you just go down with a single part flexible or if in doubt a two part flexible
By the time you have paid out for the backerboard, adhesive, screws, tape, more adhesive and then labour it is either costing the customer a fourtune or you cant be making much money on it.
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For your situation Vivaro you can't go wrong if you do what Dh says, much cheaper and quicker to use a leveller.... plus there is no risk of accidently screwing through an old pipe or something
TJ