I use sugar beet too. I have found that the sows refuse it once they've farrowed though.
I use about 20% for growers - it helps sweeten the meat. I believe that if you use too much that to 'goes to the belly'.
Theres a good guide that you can link to from this page
http://www.britishpigs.org/feeds.htm by Professor Sandra Edwards of Newcastle University has produced a handbook for feeding organic pigs which has recommendations on the use of Silage, Sugar Beet, Potatoes, Brewers Grains etc.