good point. you should be confident with something like

-log(4x10^-3)

since this is right from the pH eqs. remember:

-log(4.0x10-3) = -log(4) + -log(10^-3)

= -log(4) - (-3)

-log(4) is another way of saying 10^x = 4. x=?

well, 10^0.5 is around 3.2, so x must be higher than 0.5, like 0.6.

so -log(4x10-3) = -0.6 + 3 = 2.4