about ntfs on-disk format

hi~all
how can i search a file on the ntfs partition?

i open a disk, and read some informations from boot record of the partition...then, i get the dot dir position and read the index record, then fetch the file reference of each file in dot dir from the index record. finally, i find the file record from base mft indexed by file reference with fallowing formula.
file record = mft + (file mft record number) * 2

the question is that i will get a wrong file record from the formula, why? where am i wrong?

i found an interesting thing, when i get i wrong file record, the FILE record number and Sequence number is not equal of the file reference
how can i fix the problem. thanks~~
jinhao