I don't know if that freewebs.com version of
Critter's Site will be updated anymore, but it seems to have the same upgrade formulas for now, so it doesn't matter much. The formula you are looking for is the 3rd or 4th in the "
Upgrading Items" list found on the site you linked to. Just keep in mind that upgraded items can often have pretty high level requirements. I think the new level will just be something like the base-item-level +8. e.g. Upgraded Death sash or glove is lvl 48.
Some of the recipes on the page might not be accurate, for example the Horadric staff recipe requires Horadric ore in addition to the ammy and staff. The Rube Upgrade recipe to make an Amn rune probably doesn't require a chipped topaz.