Say if you wanted to stretch the upper right 1/4 of the texture to full uv range:
you'd put repeat to 0.5 0.5, because this repeats the lowest left quadrant we put offset to 0.5 0.5
So the formula is as follows
Get the precentual width of the area you want to stretch put that in tile, and put the precentual distance to left corner of your sub texture to offset
say your texture is 1024*1024 and you want to map form pixels 600,600o 800,800 (calculated form lower left corner) that's 200/1024= 0.1953125 So that's whet you put in repeat. Now your offset is 600pixels wich is 600/1024=0.5859375 That's what you put in offset
Incidentally if you ever need to tile a sub region you take this idea one step further you connect a placement node to placement node.
PS: If you find this hard to visualize make a nurbs plane and use the interactive placement on temporary plane to get it right because now you have nice handles at all (YES IT MUST BE a nurbs plane)
