[QUOTE=markbarendt;1192835]Ansel's formula requires a reading. No reading, no answer.[QUOTE]

You are right. I gave Ansel's formula another reading. He only gave one example where the moon was 250 candela per square foot. Now that is a reasonable constant brightness that could be worth memorizing.

And he reminds that estimating exposures from memory like this and Sunny 16 are emergency measures unlikely to result in a fine print.

So taking meter readings is important.