Sealing replacement wood door cill

Am about make a like-for-like replacement of an external door frame with a hardwood cill (only the frame - the door is OK). the door opens outwards. The old frame has suffered damp rot problems around the cill - possibly partly due to inadequate sealing around the cill.
A row of blue bricks runs under the cill. On the indoor side, the concrete floor level finishes about half way up the cill. What I'm condidering doing is to lay a bed of sand-cement (1:3) mortar on the blue bricks, then lay a pvc dpc on which to stand the cill/frame.
Is this the right method? Would a layer of mastik over the dpc help prevent water penetration? & if so, which mastik?
TIA
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