Fritzo, you are correct. The scale is stupid because it varies from person to person. The scale is defined as:
SPF = (the amount of UV radiation needed to cause you to burn with the sunscreen) / ( the amount of UV radiation needed to cause you to burn without the sunscreen)
So one SPF may be perfectly fine for you for a certain amount of time outside but the same sunscreen probably won't be good for other people. That is a stupid scale in my opinion. (And that isn't even going into the choice of a narrow wavelength range for the SPF measurement, since people are exposed to a much wider range in the sun).
The key to remember is that a thin white T-shirt is SPF 7. Anything with SPF over 7 is better than some clothing. But there is a major catch. To get the real SPF, you have to apply 15-30 minutes before you enter the sun, you have to reapply before 2 hours have elapsed, you have to apply at least 1 ounce of sunscreen, and you can't have it wear off due to things like swimming.
Most people fail miserably at those application rules, and thus most people think they need a high SPF. In reality, they are just applying a high SPF in a very poor way and getting a fraction of its real SPF. The result is usually an odd array of burned / unburned areas depending on how well they applied in those locations. If they just applied SPF 15 properly, they'd be good in most cases.
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