=IF(D3>0,LOG(C3/F3),">"&LOG(C3/10)) gives me the two options that I want.
Thanks to all who responded. Now, the first part is rounded to 1 decimal
place, ie 0.0, because I chose so in custom mode. When the second part is
true, however, I loose the custom formatting and get several decimal places
added. How do I prevent this?