We get really different results from the leaf number depending on how much rain we get in those last weeks. Sometimes you have six leaves looking green, but the tips are brown and when you dig you find most of the wrappers are gone. One way around that I've been told to estimate 1/3 of leaves remaining based on the total leaf area and parts that are drying down. But we still can get more green than that even though they're ready. I don't like to see any of them get down to less than 4 leaves, although the Purple Stripes often seem dry before they are due to harvest.
The other rule of thumb I use, is that they should be ready to harvest around 3 weeks after harvesting the scapes. This year the three week rule was spot on. They all had well formed cloves and I had just a couple - one each in different varieties - that had gone to a split wrapper.
Some people will leave a few scapes on, and they say that when they stand up straight, they're ready to harvest. I left one on my porcelain this year but it wasn't straight up when I harvested, and it still isn't - don't think it ever will be. Sitting out there in the field, maturing crooked. So much for signs.
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