[pHz]

its fine to use the longrod

but

first check the longrod is straight by holding something straight (i use an arrow) against the sight window and checking it thats parallel with the longrod - if it is then using the longrod is OK - if its not then you have to take note of the angle at which your longrod is out by and compensate for that when checking alignment

(IIRC this is the method outlined by simon needham)

slainte rob