GIMP :: How To Exactly Place Color In XCF File

I'm starting with a new file on ofnuts recommendation...see "Edit GIF image file"

I have duplicated a GIF file from a website, so I can make changes and replace it. The new file I will be exported to PNG, GIF, TIF, or JPG when complete. It's a horizontal rectangle; the leftward white 2/3 has text I will change, the rightward 1/3 is all green. I have an XCF file that I copied from the GIF as a working file, and the new file, created from scratch, which now contains my new text. The new graphics must fit into the webpage and line up exactly.

I had "selected" the green section and pasted into my new file, but it was not completely left justified, like the original, so I slid it over with my cursor, but apparenlty "dropped" it too high, because when I place the new JPG file in the webpage, there's a little white line at the bottom of that file, and it needs to butt up against the green right below it. ie, no white space.

Rather than dragging and dropping imprecisely with my mouse, is there a way to place the green area with some sort of calibration tool? Like "by the numbers" or snapping it into place?

Is there a way to "read" the green area in my working file, to see the precise size?