One trick with the clamp meters is to zero them while in the vicinity of the wire(s) (orientation and location as close as you can get it). They work by measuring the magnetic field created by flowing charge, and ambient field can produce a non-zero reading even with no wires enclosed within the clamp.
You should be able to measure both wires at the same time as you show there, or one at a time, and add them - and get the same result either way. And as said before, the negative cable should show the same number (the battery does not actually store electrons, it stores the energy chemically).
Main thing is to get things figured out so you have reliable numbers to work with.