Tamiya Ral 7028 mix?

XF-78 is a good call, I think. I found XF-60 to be a bit too dark, though I did read a while back that the "dunkelgelb" used for basecoating armor was darker than that used for camouflage so depending on what you need it for, you might be able to get away with XF-60. 

Or you could just go with XF-78 for a more "what you'd expect RAL 7028 to look like" shade.

Thank you!

If you don't mind changing to another paint company, you can try Model Master's Acryl Dunkelgelb.

Good call on the Model Master! If you ever do any interiors and don't feel like mixing an elfenbein, Model Master makes a "Panzer Interior Buff" which looks pretty good to my eyes.

Vallejo Air also makes a nice dunkelgelb (and an elfenbein, come to think of it), that's what I've been using for a while now. Just note that the Vallejo dunkelgelb surface primer is the darker basecoat shade, and is not the same as the regular Vallejo Air dunkelgelb.

Also Mission Models makes an early and a late,very nice to spray

Thank you everyone!

been experimenting and came up with these Ral 7028 mixes:

#1 XF-57 (Buff) 80% / XF-49 (Khaki) 20% 

#2 XF-60 (Dark Yellow) 50% / XF-57 (Buff) 50%