You're over-thinking this, and also trying to translate road bike measurements to a mountain bike. The two bikes are very different, and hopefully you'll also be doing different types of riding with them.
You mentioned that on a mountain bike you drop the saddle a couple of inches. What's your existing mountain bike set up like? What bike is it? What sort of riding do you do?
If you are worried about spacers and stem position, bear in mind that you can use higher rise stems and different rise handlebars to tune the fit, and also flip the stem - you'll have two different positions that way. Stem length will also be a factor. Basically, there is plenty of scope for variation by simply trying different bar and stem combinations.
Bear in mind that On-One tends to go for a slightly longer than average top tube. Your Lurcher is the same as my Scandal 29er (I ride a size small, and it sounds like you're more M or L sized), and the recommendation is for a 50-70mm stem. With Rebas, a 60mm stem with one 5mm spacer underneath and flat bars, the position is perfect for me. Maybe try a cheap stem in the 50-70mm range (or whatever is recommended for your Lurcher size) and then try different riser and flat bars to see what works for you.
By going the carbon steerer route, you are rather limiting your options to get the right fit. Why not consider buying a cheap second hand pair of rigid steel forks the same length, and get the position right on those, then cut your niner forks to fit once you're totally confident? Probably a bit academic - I imagine with all that lovely kit begging to be built into a bike you just want to go and ride it.
Just remember - you can always cut a steerer down, but you can't add more back on to it.