First:
It's not enough to have max smithing skill, you also need to add perks to alchemy and enchanting to really maximize the utility of an armor set. Add max perks to basic enchanting and alchemy (i.e 5/5 - five perk each basic skill). This lets you make som strong potions and enchantments. Remember to have the "benefactor" skill (under alchemy upgrade). In addition, it is useful to have the "insightful enchanter" skill under the enchanter upgrade.
Next, do the following:
SMITHING and ENCHANTING:
2 rings
2 neckleses
2 armors
1 Bracets/gloves/gauntlets
ALCHEMY:
1 ring
1 neckles
1 Bracets/gloves/gauntlets
1 helmet/hood
Any kind of these items will do. Just collect the cheapests.
Now, enchant the alchemy item set above with fortify smithing and make sure you have filled grand soul gems.
Next, use the new enchanted items and make fortify enchanting potions and fortify smithing potions.
Now, use the fortify enchanting potions when enchanting the smithing set. This will give you 1 ring, 1 neckles, 1 armor and 1 gauntlets with 28% better weapon and armor improvement. Remember that you also have the smithing potions in addition which add 127% better weapon and armor improvement as well.
When you have improved the armor you can use the enchanting potions to put som powerfull echantments on the armor (offcourse, the nightingale set already has enchantments, but you probobly find other sets to improve and make better)
I give som examples here on my improvements:
The total armor rating is 690 with Nightingale set and 1200 with the dragonscale armor (that includes the full set of lightarmor bonus)
Nightingale set
http://i612.photobucket.com/albums/tt210/andreasmoen/Nightingale-armor.jpg
http://i612.photobucket.com/albums/tt210/andreasmoen/Nightingale-boots.jpg
http://i612.photobucket.com/albums/tt210/andreasmoen/Nightingale-gauntlets.jpg
http://i612.photobucket.com/albums/tt210/andreasmoen/Nightingale-hood.jpg
Dragonscale set
http://i612.photobucket.com/albums/tt210/andreasmoen/Dragonscale-armor.jpg
http://i612.photobucket.com/albums/tt210/andreasmoen/Dragonscale-boots.jpg
http://i612.photobucket.com/albums/tt210/andreasmoen/Dragonscale-gauntlets.jpg
http://i612.photobucket.com/albums/tt210/andreasmoen/Dragonscale-helmet.jpg
I havn't encountered a higher improvement level than "legendary"