Hi WD_GASTER2,
Assuming that the NAND size is 2GB and using the steps from mdfbrew (End address for 2GB NAND is 400000 hex), the missing value (count) should be 0x003CE800.
If you are using Windows 10 you use Calculator in Programmer Mode and subtract 31800 from 400000. The result will be the block count for the data partition.
EDIT: I think I misread your messsage. Is the NAND size really 256MB? If it's 256MB, the missing count should be 0x0004E800.
256MB = 268,435,456 bytes / 512 (bytes/sector) = 524, 288 dec = 0x00080000 hex. 80000 - 31800 = 4E800.
let me atempt after dinner. ill give some feedback soon