You're in luck, because I have exact numbers from doing this very recently. Go to the tannery, right across from Lord British's Castle/Britain Library and purchase a set of studded leather armor.
Stealth Gains
0-60: naked
60-68: wear studded arms
68-80: add studded legs
80-100: add studded tunic
Past 100 I have not done recently, so no guarantees, but wait til you stop gaining with the tunic. Then add the gloves/gorget. Finally, you can use a close helm purchased from a blacksmith for any stubborn last gains. Stealth is slow, so put on the one piece and wait, wait, WAIT.
Note: Never tried this with player crafted armor, so buy it from the NPC.
