SPF levels in homemade sunscreen can be tricky, but as a rule of thumb zinc oxide should make up 20% of the recipe. If you are fair-skinned, use an extra tablespoon or so to up the sun protection factor. This recipe yields an SPF of around 20, protecting against 93% of the sun’s rays. It is important to note that moderate sun exposure plays an important and healthy role in converting sunlight to vitamin D in the skin. So, spending small amounts of time in the sun is beneficial.