The formula for determining the ilvl of the amulet made is (.75 * Character level). Hopefully this information helps someone else out there who also had this issue a few times. I eventually tried again in the future and (most of the time) was able to open a portal. I have had situations where the Cow Level portal does not open for me even after defeating Baal and logging in/out of the game. The exception to this rule is if you're using the PlugY mod which allows the portal to stay open no matter what. Additionally, if you defeat the Cow King inside of the Secret Cow Level you'll lose your ability to open the portal.

In order for this recipe to work you must have killed Baal on your current difficulty and you also must be standing in the Rogue Encampment (Act 1). Wirt's Leg + Tome of Town Portal = Secret Cow Level Horadric Cube Upgrading Weapons & Armor Horadric Cube Gambling for Socket Recipes If the recipe you want is not on this page then I recommend you follow one of the links below and check out my other Horadric Cube recipe guides. For the most part I have the most commonly used/best recipes listed at the top of this page, the less useful ones are further down towards the bottom. Any recipe that I felt wasn't worth mentioning on the other pages or doesn't fit into the category of the other pages on my website can be found here. On this page you'll find every misc recipe that can be created with the Horadric Cube in Diablo 2.

Horadric Cube Miscellaneous Recipes - Diablo 2

Diablo 2 cow level recipe