What the Bible says about Measuring the Altar
(From Forerunner Commentary)
God next speaks of the "capstone" or "headstone." Some people have assumed that this was a foundation stone, but it is not. God is speaking about finishing the temple, not starting it. This is not the cornerstone, which is installed in the foundation, but the finishing stone—the very last one set. It can also be called the gable stone or even the keystone that would finish an archway. It is a symbol of completion.
In this case, it represents the Temple being sufficiently ready for God's habitation. This ought to be clear. He is alluding to the preparation of the church for God's Kingdom. He is being encouraging, not only to Zerubbabel, that he would finish the Temple, but also to the Two Witnesses far in the future, that their job of measuring the Temple, the altar, and the worshippers will be successful. They will be successful by "grace, grace"—double grace. Their work will be accomplished only by an extra measure of God's grace.
Richard T. Ritenbaugh
The Two Witnesses (Part Four)