What backing for braid?

I am going to be changing over 4 of my reels that are for deep drop from 100 lb mono to a 130 - 200 braid.

I'm interested in only putting 300 yds of braid on these reels. I am expecting I'll have 2\3 rds of the spool empty with out a backing.

Since I don't expect to expose the backing.

What are my options other than leaving a portion of the origional mono on the reels?

These rigs are not for jigging but for sending live baits to the bottom.
Thanks in advance.

3-30-07 on the Big E