How's the top tube length on the bike compared to others you've had experience with? Are you looking to be in a similar position or more upright?
Riv recommends starting with an 8-10cm stem, but it helps if you know the kind of reach you're looking for.
Even if you come up with an idea on your own, it wouldn't hurt to call/email Riv and ask for their recommendation for your 53cm Atlantis based on your height and PBH.
For an idea of relative sizing vs drop bars - I recently converted a drop bar bike to Albatross bars and moved to a 40mm longer stem. This is a sporty roadish bike and the grip position on the Albatross bars is more upright than I ever could be on the tops of the drop bar (but still not a very upright position) with the bends being about the same reach as the hoods were before.
A note on shifter placement - the bike mentioned above is shifterless and I enjoy being able to put my hands anywhere from the grips to up at the bends to vary the reach. If I were putting thumbies on these bars I'd put them up inside the bends so as not to spoil that. My Platypus has Toscos setup in a similar fashion.
That said, I don't mind moving my hands to shift. If you have no desire to compromise shifter access for a bit more hand position flexibility then ignore this.
-Dave