What % native urea gel do I use for Fab and F(ab')2? - (Jul/09/2012 )

I have an Invitrogen Xcell SureLock mini cell for gel electrophoresis.
As far as SDS-PAGE gels, I would use a higher % gel for lower M.W. protein.
My boss asked me what percent gel I should use for Fab and F(ab')2.
I suggested 18% for Fab and 12% for F(ab')2 for a SDS-PAGE gel.
He says it is not quite correct.

I am thinking it is because he is looking to use a native urea gel .
What % native urea gel should I use and why?
If you could link the Invitrogen gel cassette for the gel you are referring to, that will be helpful as well.

urea enhances charge separation of otherwise equal proteins (the more negatively charged protein will migrate farther).

technically, urea-page is not native (urea does disrupt protein conformation) but you add urea to native page (similar to adding sds to native page).

we used a 7.5% acrylamide gel to separate phosphorylated from non-phosphorylated 20kDa proteins and 3.5% (i think, may have been 5%, sorry, it was a long time ago) for 200kDa.

Thank you for your prompt response.

Does anyone else have more information to add to this topic ?