Originally Posted by
laxmillenial
It's not always correct though.
I booked LAX-MEL-LAX and LAX-SIN-LAX. GPUs on both directions (separate trips). IN was 0 for all segments. R was 9, 1, 3, 1 respectively. Your theory is decent, but you risk cutting yourself off of flights that might have space, mainly the R=1-4.
Yep, I get that it will miss something. But the OP (and many others) don't wish to look day by day as you and others might. It's a helpful tool for a quick search as long as one understands the limitations.