Worked example: age 40 household
The default example uses age 40 and household net worth of $250,000.
Age 40 maps to the 35 to 44 SCF age group. In that group, p50 is $135,300 and p75 is $415,000.
$250,000 is $114,700 above the median and $165,000 below p75.
Linear interpolation between p50 and p75 gives 50 + ((250,000 - 135,300) / (415,000 - 135,300)) x 25 = p60.3.