what is the formula for rippling a voltage or filtering a capacitor about 10%

 

Some info here...use a search engine, also try "designing power supply" to find more streamlined information on doing this.
I know there are 1 or 2 eqn's to set up which C to use where you can set your own percentage tolerance, but don't have that reference handy...

https://en.wikipedia.org/wiki/Ripple_(electrical)

 

See this site:

http://www.zen22142.zen.co.uk/Design/dcpsu.htm

 

You may also like this post which appeared in this thread.