Nikon SB-28 - Manual (Page 57)

57
For film speeds other than ISO 100, multiply the guide numbers by the factors shown
in the above table.
For example, if the film speed in the previous example was ISO 400 rather than ISO
100, the guide number is 72 (36 x 2).
ƒ
and
"
flash modes
Adjustment factors for other ISO film speeds
ISO film speed
Factor
25 50 200 400 800 1600
x 0.5 x 0.71 x 1.4 x 2 x 2.8 x 4