I started on a factor 30 last year (I tan quite easily) and after a few days dropped to a 15, not a good idea.
My shoulders were pink within an hour of sitting on the beach.
The sun in
icmeler is very strong, and the cooling breeze you get on the beach makes it feel a lot cooler than it actually is!
Use 30 for the first week would be my advice, and then drop down to 15 if you've got a nice base tan going on.
Chris & Anita