Check a cross-section under the dissecting scope. Cyanobacteria look blackish. This is an occult art. Many lobes swell or fork at the tips. Measure behind the swollen or forked area.
Some thalli have lobes of different sizes.
When this happens, you have to guess what the author of the key has in mind. Hale suggests averaging. I usually find it best to use the smallest available well-developed lobes, but sometimes the opposite is needed. Scallops, indentations, and incipient lobes don't count. Both are important.