Lightroom Relative adjustment plug-in or preset
Do you want to add a little extra brightness or ... to multiple already edited photos: "Relative adjustment" is your choice.
Do you want to apply a preset stronger of softer: "Percentage adjustment" is your choice.
Do you want to apply develop changes only on new virtual copies: "Appy to Virtual copy" is your choice.
The "Relative adjustment" plug-in can do all and even combine these functions.
Lightroom Video Tutorials
Mastering Lightroom: How to Manage Presets
LR3 - Making Relative vs Absolute Adjustments to Images in Lightroom