I will take a stab at this.
Assuming your balanced equation is right,
150mg * (1g/1000mg) * (1mol KI/ 166g KI) * (1mol I-/ 1mol KI) * (5mol I2/ 10mol I-) * (254g I2/ 1mol I2) = .115 g = 115mg I2
108.5mg is what you obtain experimentally and 115mg is your theoretical amount, so 108.5/115 = 0.94 or 94%
94% is your percent yield