Sunscreen SPF and Skin cancer

It isn't easy to answer because it really depends on how sensitive your skin is. Usually people with lighter skin are more sensitive than people with dark skin.

In general the more protection the better. 

There is no simple answer to this - it all depends on your sensitivity to the sun and whether you want a tan or not. The main problem is that for the best of health you need to get some sun on your body without using a sunscreen of any kind. The sun is essential for you to make vitamin D. Lack of this vitamin is linked to many diseases including cancers. So 20 minutes a day of sun is thought to be what you need, without burning your skin. Then you should cover up either with clothes and hat or a sunscreen of high factor (15 and above), or go indoors or in the shade. And, there is a school of thought that says there could be a link between the chemicals in suncreens and some forms of cancer. So suncreens are not necessarily the answer to good health.

Any  Sunscreen  over 30 SPF is a waste of Money.

Use 30 at least and remember...you have to reapply frequently. The tube should give you instructions on reapplication. Many people mistakenly feel that one application will do for a day of sunning...it won't. Be safe.