Hard data is nice to have IF you can get it but sometimes its not possible for the average person to collect hard Data for every single situation, that's where the math formulas, data cards and apps come into play.
I know/worked with plenty of Snipers that use their apps first and place the confirmation info into their data book.
Everybody has their methods.
I use the apps to generate the solution I need, then confirm it ASAP. I'll oftentimes work out the data the "long" way, then verify it with an app, then lock it in with live-fire. I like to keep my skills up, in case I can't use the app.