Lee, I have 90mm 6 degree stem and want to convert to a 70mm. What rise do I
need for the height to be the same? I was thinking 10 degrees. thanks
Yes, 10 mm.
A 90mm x 6 degree stem has about 7 mm of rise. A 70mm x 10 degree stem would have about the same.
As you shorten your stem, you usually want to increase the rise. Imagine an arc that sweeps down and away from you; you want your bars along that arc.