I adjust the line spacing pretty frequently, and it's really difficult to get it to a precise level.

The slider will almost always shift once you lift your finger or pencil from the iPad. This same problem also happens if you're using a Mac with a trackpad. It wouldn't be as big of an annoyance if the + and - buttons adjusted the slider at a better rate. On the Mac, one click on either button only adjusts the value by 0.01 pt.

Personally, I think it'd be a lot easier for the user if you could manually enter the pt value. For example, you could start off with the slider to get a rough estimate. Then manually enter a similar value to get it a little closer. After that you could use the + and - buttons to get it right in perfect spot