I have tryed the chart . It does work . I found an anomoly in the math . Here's an example . I got this 14+11 = 25 , 2/25 = 12.2 . If you get a .2 as in 12.2 your your child would be an 13 not 12 . anything over .1 to .9 , just move up one on the chart .
If this has been posted before I'm sorry, I didn't check every post .