I don't know, but the thing you want to look at on the sun tan lotion is not only the factor, but the UVA rating which is even more important. It's shown as a circle with UVA written in it and up to 5 stars - look for at least a 4 star one, and 5 stars if you can find one. If you get a waterproof one, it's probably only a question of slapping it on 2-3 times a day, and then forgetting about it. I did hear someone say you are at more risk if the RT caused any skin marks/darkening/reddening/irritation at the time - mine didn't, but it was only 4.5 weeks.
Obviously, keep an eye on the area.
Also note the sun goes through most thin T-shirts/vests, so you will still need it on under one of those.