Changing shiny odds to match later gens' odds?

[Rei13]

I've been looking around the site and I know what offset needs changing in order to modify the odds. However, from what I could find, people have been saying to replace 08 with FF, which gives 1/256 odds. I'd like to change it to match the newer gens' odds. I'm very much unaware of how hex editing truly works, so I have no idea, if FF means 1/256, what I need to put in to result in the odds I want.

I added this on the NDS section, but really, I'm interested in Gen 3 as well as 4 and 5. But since there are more games on the DS, I decided it'd make more sense to put it here.