How much height is added by flipping a 6 degree stem?
 

I've got a 100mm 6 degree stem at -6. By flipping to +6, how much higher will the bars be?

2cm.

You might end up spending more time in the drops though.

Plus about a 6.5 mm reduction in reach.

I just rebuilt a bike and the stem is 6 deg. by 100mm. I first tried the stem with a 6 deg. drop and thought maybe it was a bit too low for me. The next day I flipped it for 6 deg. upward. I knew ahead of doing this that I like the looks of neutral angle on a stem but am more concerned about the comfort/fit. I did not like the look or my position on the bike with the upward angle and switched back to the negative, but with one more spacer below and one less above the stem. It just works better for me. I think trial and error is the way to find what works for you.

90 degree stems are hard to find, but that's as angled up as you can go on a road bike before it looks off... Makes a decent amount of difference from a 6 degree stem though.

The cool thing about them is it's the shortest path from bar to steerer, so it kind of makes the most sense, especially if that means no spacers. Of course, there's no point flipping it, which might be considered a downside...

Easton makes a couple:

Easton EA90 Stem 0 degree | The Colorado Cyclist

Easton EA70 31.8 Stem 0 degree 90mm (planetcyclery.com)

Exactly! The body is often able to adapt to small changes. Even mine, although I have not touched my toes without bending my legs since high school, which was a long time ago. So document and experiment, one small change at a time.

For example, I recently flipped my stock 100 mm stem from +6 to -6 degrees. I must admit that this change was driven mostly by aesthetics and slightly by curiosity, but if worse comes to worst I could easily reverse it. Unexpectedly, I got used to it within a couple of rides. I figured I could go a little lower so I bought a new 100 mm, -8 degrees stem. The new stem turns out to have a higher stack height than the stock one so I had to remove a 5 mm spacer to install it. So now I am even lower than anticipated (from just a -2 degree change). But I got used to this new setup within a few rides too.

I really like the Velo-Orange and Soma Tall stack stems, 75 mm stack compared to approx. 40mm for the usual threadless stem. They give you some extra height, but with the 17 deg. angle it makes the stem just about parallel with the surface the bike is on. However, they are not recommended for forks with carbon steerer tubes.

Yeah, like I said, not many out there - the 90 degree stem I have is one of those.

h = height of the triangle side opposite the 6 degree angle

vertical displacement from flipping stem = 2 * Reach * Sin(6 degrees)