Is there a way to configure a Steam Controller while playing a video game?

I recently bought a steam controller and played Witcher 3 with it. So far, it's good. I like the controller. But I have some troubles to press XYAB buttons because my hands are too small to reach them.

Anyway, so I tried to change its default settings for Witcher 3 on a Steam Controller configuration page in Steam Big Picture, but it's pretty hard because I need to choose a key or mouse button to assign to a controller's button or so. That means I need to know which key goes to which action in advance!

Well, I googled Withcer 3 key bindings and found the following picture, so I can probably set the controller up with it, but still when I want to try my configuration, I have to exit from Big Picture then restart the game to test it. If I don't like something or make a mistake, stop the game then start Big Picture.

https://www.reddit.com/r/witcher/comments/3690ue/witcher_3_pc_default_keybinds_what_will_you_change/

How do you guys try your settings on a controller usually? Do you set it up, and start a game to test it? Or any other better way to do it? Ideally it would be great if you can set a controller up while playing a game.