I've currently got ~5.75" / 146mm simply because I can't lower the bars without replacing the stem. I started with the stem higher and kept going down to see where I was comfy and ran out of room. At some point I'll get a -25 to -35 degree stem and see how that feels. Gotta say, though that I now ride around 90% of the time in clip on aerobars. I ride down in the drops only when in a killer headwind or situations that need the handling finesse of the wider position.