Remember "live", we are only talking about the nitrifying bacteria on/in the rock, much like live sand. 20% could be a decent number, but if you are cycling your tank, I imagine it wont any drastic effects either way. The critters on the cured LR
/live sand are just bonuses.
I think it will just take longer for the colorful coraline to move onto the base rock if you have a small amount of LR
I have been in contact with Marc, from marcorocks.com/ and plan on ordering from him this Friday, for my base rock, I'll pick up the LR
as I find pieces that I like from my LFS