Not having created a spreadsheet for in-house use, I use pencil, paper, and a copy of Skunk Works on my desk/table/sofa. I do have the basics (Signature SP/HP limits and PV, movement and armor costs, basic weapons calcs, and basic infantry costs) memorized, so I can usually knock out a vehicle design (provided I have the mission profile and TL defined before starting) in 2-5 minutes. Finishing the (book) layout for a stat cards takes longer than the actual creation of the unit.
While I don't expect many others would take the time to memorize even this (relatively) small amount of information, you'll remember more than you think after completing a couple dozen designs (and definitely after having done 600 or so).