UPDATE 2-Ex-rebel Prince Johnson backs Sirleaf in Liberia runoff

* Johnson is senator from populous Nimba County

* Says supporting Johnson-Sirleaf is "lesser evil" (Adds final tally from Oct. 11 vote in paragraph 4)

By Alphonso Toweh

MONROVIA, Oct 18 (Reuters)- Former rebel leader Prince Johnson, placed third in the first round of Liberia's presidential election, said on Tuesday he would back President Ellen Johnson-Sirleaf in the Nov. 8 run-off against Winston Tubman of the opposition CDC party.

"I will support Madam Ellen Johnson-Sirleaf in the run-off election," Johnson told Reuters by telephone.

"This is because some of her policies are good for this country. If all her policies are not good, we will do addition and subtraction so that what we want to see in it, will be reflected," Johnson said.

Results announced on Tuesday showed newly named Nobel Peace laureate Johnson-Sirleaf scored 43.9 percent of the vote, ahead of Tubman with 32.7 percent and Johnson with 11.6 percent.

Observers have praised the peaceful manner in which Liberia's second post-war ballot was carried out. The vote is seen as a test of progress towards stability and the country's readiness for investment in untapped mineral and agricultural resources.

Johnson, who was filmed watching his fighters torture former President Samuel Doe during Liberia's civil war, is now a senator in Liberia's northern, mineral-rich Nimba County, the second most populous in the West African state.

Analysts have said that Johnson, a former member of Sirleaf's ruling UP party, was most likely to side with the incumbent because her international profile could help him to consolidate a legacy as a revolutionary-turned-politician.

Sirleaf, a former World Bank economist, has earned international plaudits for maintaining stability and reducing debt in Liberia since becoming Africa's first freely elected female head of state in 2005.

"If you have two evils, I will prefer to go with the lesser evil. In this case, the lesser evil is the incumbent," Johnson said.

"I think I am prepared to support a person who has six years to be in power, rather than to go with a person who will go for twelve years," he said. (Writing by Bate Felix; Editing by Philippa Fletcher)