The percentage of households able to afford a home in Los Angeles County fell even further in the first quarter of 2007, according to data just released by the California Association of Realtors. The group reported that 20 percent of L.A. County households could afford an entry level home, down from 21 percent in the comparable period a year ago, but up from 19 percent in the fourth quarter of last year. The C.A.R. index is based on a median price home of $501,390 and a monthly payment of $3,370.