I would like to put some cheap mono as backing on my flipping/frog rod to save some braid, but I'm not sure what lb. test I should use (or if it even matters).  Any suggestions on how what would be best?  Thanks in advance for the help!

I try to match diameter to diameter. Has not steered me wrong yet. 

I try to match diameter to diameter. Has not steered me wrong yet. 

^^This^^ ... Or you can use slightly smaller diameter mono (I like Zebco mono from WM or Dicks) ... Spool on about 25 turns of mono (baitcast reel) or approx. 50% (spinning reel) with a double uni knot to Braid or FC top coat and your good to go !

Thanks to both of you!

I use 12 lb. mono for every reel.  Seems to work well for me, regardless of the type of main line that I use.

Even though you probably won't reach the backing I wouldn't go to thin.  I would not want to match the diameter of 10 or 15# braid which is the heaviest I use.  Not only would I need more 2 or 4# mono if I were matching my diameter but I wouldn't have the strength if needed.  If I have a fish stripping enough line to get to the backing 4# mono ain't gonna work to well.  I'd go 10#.

I fill the spool up with the line that I will be using.  When it is time to change line I leave some of the line on, and just put the new line over it.