Up until now I've been building non-dished wheels for my bikes. Managed this by using single speed hubs and internal gears. I'm in the process of building my first dished 20" wheel that will use a 7 speed cassette.
With the non dished wheels calculating spoke length was simple. Any spoke length calculator on the Web gave me good resuts, albeit almost always the spoke was a wee bit too long (a millimeter or so).
For th dished wheel the results tell me I need 191 mm on the non drive side and 188 mm on the drive side. When verifying these spoke lengths (with the 6 spoke method) I've determined that on a 20" wheel this will give me significantly less dish then I need to center the rim. The spoke sizes I determined I need are indeed 197 mm and 178 mm. That's a 19 mm difference in length! I want to ping this off of you whether this is a reasonable result.