Hi and welcome, Kim. It would help to know where you are, to diagnose what might be afflicting your lemon tree. Please fill in your personal profile with your location? It's the little person icon up in the blue side bar to the left on the home page.
It doesn't look too much like scale to me. The spots are too random and different sizes. Also, scales on citrus usually are on the stems of the newest growth, not on the backs of the leaves. So I think it is some sort of fungal infection.
The fungicide you've been using will most likely prevent any spread of the disease, so keep on using it according to the label directions. Remove any leaves that have spots once they start to turn yellow and throw them away in the trash, don't compost. Green leaves are still nourishing your plant even if they are spotty.
These sorts of fungal spot diseases are very common if you live somewhere hot and humid, and rarely cause too much damage to the plants, but they sure don't look good and you will lose leaves. So it's probably a good idea to begin using a fungicide regularly as soon as you see spots starting. Once the weather is drier and cooler you can probably ease up on the fungicide and see if it recurs.
