with all due respect , these are the data needed, I believe your tool can easily output them, why - since you have the possibility to provide them - you provide different units?
1) if the file is fragmented, output "File is fragmented" or a similar error message
2) the offset (from LBA 0, i.e.\\.\Physicaldriven) to the beginning of the file epressed in sectors
3) the extent (from the offset above) i.e. the size of the file expressed in sectors
4) optionally the offset from the beginning of the partition/volume (not really needed)
5) anything else is not really needed, maybe it could be output only if a -v (verbose) parameter is given.
Try using grub4dos and use its blocklist command, that is what would be useful.
For the record, there is a tool similar to getfileextents (that actually works "better" in my experience) , myfragmenter.exe part of the mydefrag tool, that outputs data in Vcn's (or clusters) on NTFS, example:
MyFragmenter.exe -i D:\Win7PE\log.html
MyFragmenter v1.2, 2008 J.C. Kessels
Commandline argument '-i' accepted.
Extent 1: Lcn=27119138, Vcn=0, NextVcn=1367
1367 clusters, 1 fragments.
Finished, 1 files processed.
but - since you are writing it - why not saving us the need to make calculations/conversions?