I always liked the look of the gauge setup in the Cessna 172 and 182 so I knew I wanted to keep as much of this look as possible in my design.
I started my design by using a template of a instrument panel from another website of a Cessna 182. This gave me the overall feel for the panel as well as the gauge sizes.
I took drawings of actual Cessna 421 cockpits and worked out the sizing of each element.
I also bought various parts from actual 421 aircraft and used these to size the MIP.