The tool sets it to ascii to the lower left. I just do my edits using a text editor
The header section below shows the number of rows and columns and the lower left X and Y values, the cell size, a nodata value... followed by the data. I I want to make it the 'center' I just move the XLLCORNER/YLLCORNER by 1/2 half the cellsize in the direction needed to put the spot in the right location. Then when I am done editing (did I say only on a backup), I then use the tool to make it a raster.
NCOLS 480
NROWS 450
XLLCORNER 378922
YLLCORNER 4072345
CELLSIZE 30
NODATA_VALUE -32768
43 2 45 7 3 56 2 5 23 65 34 6 32 54 57 34
35 45 65 34 2 6 78 4 2 6 89 3 2 7 45 23 5 ...