Drawing a black square and a white square & taking priority

I've figured out what I needed! (but I'd still like to come back around to trying to understand the <E> tag at some point...)

x = (characterCount-1)*5; //columnCount?
y = rowCount*10;
//top-left:
<P=0,0> //no change needed
//top-right:
<P=x,0>
//bottom-left
<P=0,y>
//bottom-right
<P=x,y>

So I just prepend one of the <P> tags above to my string and then follow it with <P=0,0> to reset the position of the next string that it draws, and as long my program snags the first OSD slot, I'm good to go.
I think problems will arise if RTSS is using one of the left or top corners AND I'm not able to grab the first OSD slot since I'm not move up and left with the <P> tag, but hopefully I'll always be able to take over that slot I guess.