Guide Numbers

[Jake Primgaard]

I was wondering if anyone may know how to compare guide numbers when they are given 
for different focal lengths.

For the SB-27 the Guide Number listed by Nikon is 112 @ 50mm, while the SB-28 is 164 @ 
85mm.  Is there a conversion to enable an equal comparison?  Or am I missing something?

Thanks,
Jake