I would actually take the precision of counting each square to the closest tenth of a square. For example, I'd look at a square and say: "...that's about 0.4 of a square"

One really effective way of doing this is to just count the full squares first, write that down, then try to match the incomplete squares together to build 1 (i.e: one that looks like a 0.6 and one that looks like a 0.4) and then with the leftovers, just add up the decimal points.

This is the most surefire way to get an accurate answer, and a rule of thumb like "counting it if it's showing at least half" can be very inaccurate for some curves and graphs.

I've heard they can actually be pretty tight about this (they want maybe within 10% of some number that VCAA decided was 'correct' or something), so that means you actually have to be quite accurate at square counting. With the tips above you can be accurate as well as speedy.