In what percentage of people does lhermitte's sign indicate multiple sclerosis?

Not pathonemonic. Lhermitte's sign is common in ms, but is also associated with multiple other diseases or medical conditions including vitamin B12 deficiency. You should see a board certified neurologist or your primary care doctor for further evaluation.
May be nonspecific. Lhermittes is a sign of a problem over the posterior columns of the cervical spinal cord, and is nonspecific, as can be due to ms, but also, cervical stenosis, cord compression for other reasons, pernicious anemia, etc.