When can I use a clear blue digital?

AF was due the 1st of Jan but I've been getting faint positives since Christmas Eve. I've had 6 now and on two different types of tests. On strip tests it's getting darker but still very faint. It was the same with my last pregnancy but frer picked it up a lot better. I want to do a digi so I can get an estimated at how far I am because I'm unsure if i ovulated on the 14th or 17th. Posted my test to see if you think CB digi would pick it up.