“My moisturiser is SPF 30 and my foundation is SPF 20. Does that mean I’m protected by factor 50?”
Unfortunately it doesn’t. The highest number of any one product you put on your face, in this case SPF 30, protects you. Though sometimes there can be a bit of dilution when one product is applied on top of another.
Having said that, it is still beneficial to apply two layers of different products with SPF, the second layer gives you a higher chance of getting the right amount of SPF.
To prevent premature ageing of your face, go for the highest SPF you can. And where possible, wear a sunhat and rehydrate with water regularly.