Im working through the Destroyer books and in QR and GC I'm struggling on the problems where it's required to know the answer to a more complex logarithmic function. For example it was expected I know that -log 3 x 10^-7 is approximately 6.5; or -log 9.5 x 10^-4 ~ 3. I do not know or really understand a way to estimate these more complex log functions. For the DAT do I need to know these? Or am I okay knowing just simple -log 1 x 10 ^-5 = 5 sort of things. If I do need to know these, can someone please explain how to go about learning/memorizing them?