I must have that magic touch too lol, done my first ms28 today and found the timing unimportant. I did notice this though(although I have the via8237 chipset). I tried extreme bootmaker using the latest x360 sam via a floppy and no dice. I used the same files but using a usb flash drive instead and bang.....first time.
Also dont forget the impotance of not connecting the sata cable to the drive until you reach the command prompt.....but then I guess if you got as far as you did you're pretty much there. I would suggest that textbook is correct and that it is a timing issue.....keep trying....start from say 3 seconds slowly and then work your way up until you hit the spot

.....but again.....timing was an issue when I tried the floppy method but not an issue when I used the usb flash pen
Also and again it may be different from system to system but I disconnected every single drive from my motherboard(both hdd's and both dvd rom/writers) so that my 360 drive was the only drive connected....apart from the usb flash drive ofcourse

lol
On a side note textbook just how important when entering the samread/samhack command is it too use your serial number in a block of 7 and then a block of 5 samread xxxxxxx yyyyy? I was actually successful with 2 banks of 6 lol...... samread xxxxxx yyyyyy although I obviously used the correct way I did actually try the 2 banks of 6 and everything still worked a treat? Just interested that was all lol
Help got sis180 sata control on board when i turn off 360 and choose master and then turn 360 back on it reboots pc