I always have my doc do it via LabCorp and I joined the D Action Study last August. I encourage everyone here to join Grassroots Heath's study.
Here's a ton of D information I wrote....much more thorough info and cites are available at vitamindcouncil.org
1)just what you need
2)lots of dietary antixodiant protection and topical antioxidant protection
3)saturated fat in the diet (oxidizes less easily than other fat)
10-15 svgs fruit and veggies....topical C.....consumption of green tea......topical green tea application etc
WRT to sunscreen: euro s/s containing both tinosorbs will typically give the best UVA protection in the higher ranges (360-400 nm) - the ones that most others don't cover.
You'll hear often that TiO2 and ZnO are great....in theory yes. But TiO2 doesn't protect well across much of the UVA spectrum - and while ZnO does, it's exremely inefficient. A euro screen (like the Biodermas) containing both tinosorbs and avobenzone, might give a PPD of 30 (measure of UVA protection...spf only measures UVB protection) with say....10% filters but you can have a 20% ZnO sunscreen that has only a ppd of 10.
At makeupalley.com, you can read notepads users: beethovengirl, sunscreens, sunscreendata for more scientific sunscreen info. Terakis is also a great notepad.
Carol Demas has written two fantastic books that are available on lulu and cafepress respectively: Sunscreens and Make your Own Effective Cosmetic Treatments.
Neutrogena's new pure and free baby and the adult version ("sensitive skin") appears to have boosters formulated into the mix resulting in a higher PPD with just physical filters - even my uber-quick-to pigment child isn't freckling with the neutro baby/sens skin s/s applied at proper use rates.
I stick with my beloved bioderma or avene s/s....realizing of course that they are not at all waterproof or water resisent.
This is such a problem in australia not only b/c of the thinning ozone, but because this is perhaps the largest displacement of uberwhite people living in a land for which they are totally genetically adapted.....south africa is another such snapshot.
People with high circulating 25(OH)D rarely get melanoma. Interestingly basal and squamous cell carcinomas, which don't carry a great deal of risk, are associated with a decreased risk of melanoma.
The bottom line is: keep the s/s exposure for d production 1)in the summer 2)midday 3)the minimum needed - ie long enough to be just before a burn would be caused......AND that when s/s is worn or when sun isn't sufficient for D production, wear excellent UVA protective s/s at proper use rates or cover up or seek shade or go indoors. Eat a diet rich in antioxidants and sat fats.