I'm about 6' 1.5" and would never have been happy with Xootr's longest stem (only 100mm). I use a 120mm, 17-degree stem with drop bars. That's with Xootr's XL (12") riser, and puts the top of the bars pretty much exactly level with the saddle. If I had flat bars, I'd want an even longer stem, or perhaps the same length but with Xootr's standard (10") riser (which would put the stem mounting point not only lower, but also slightly further forward).

I suspect my leg/torso ratio is longer than average, since I also needed the XL seatpost. If the Large seatpost has enough extension for you, you probably want to stick with the standard (10") riser unless you're looking for an especially upright position (bars higher than saddle).

Your LBS should be willing to let you try a few different stems, and hopefully has stems for ~$30 (I've used Dimension and Profile Design stems in that price range). You can often find surplus stems extremely cheap on www.icyclesusa.com, but it can be hard to find the exact length/angle/clamp size combo you want.

Do you have another bike you find comfortable? You could try to replicate the top tube + stem length from that, if the relative bar/saddle height relation is in the same ballpark.