So if you have a type T planet, with an oxygen atmosphere, your MI is 5 (tables 115 and 119). Now on table 122, you roll percentile for each category (Heavy Metals, Radioactives, Non-metallic, and Industrial). This gives you a range of 5.25 to 7 depending on the roll. You roll the rating, lets say the percentile was a result of Normal. Your MI is 6, and the rating is 1d10+5. Those values are multiplied together.