How much weight should I lose from 270lbs to reduce my risk of hbp and high cholesterol?

Ideal body weight. You should strive to reach your ideal body weight. This of course is dependent on your height so without knowing that it would be impossible to give you a number.
Start with 10% Start w/a 10% weight loss (so for you, 27 lb). Studies show that a 10% weight loss will have a significant impact on your long-term health. At that point you will be at 253 lb. Stay at that weight for a bit and then shoot for another 10% (25 lb). Then you'll be at 228! stay there for a while and see if you want to remove another 10% (23 lb) and you'll be at 205! just take one step at a time.