Concentration of potassium triiodide needed for titration?

I'm doing a titration project to find the amount of vitamin C in a solution. I will be using potassium triiodide to react with soluble starch in the solution. Can anyone tell me the concentration of potassium triiodide needed? And how to get potassium triiodide from iodine and potassium iodide (ratio of the ingredients, concentration, how to react them etc). Thanks!