If you want to use other elements - which can be smart if you intend to focus on Dark due to Anubis - then I don't have a proper rule for it.
I am not a big fan of it, but Sanathlig made an Accessory exp calculator. I find it quite horrendous, but it works to this day. Just remember to aim for halfway each time, since you don't want to level up SSRs in three or more parts. Halfway is 6675 exp, by the way.