2/3rds as a general rule of thumb? Nope. It depends on the flaccid size.
LPSG can't really be used to judge what the standard ranges of growing and shower are. Here is some data from a site several months back (someone else also linked to it in this thread), I don't know if more has been added since then.
Also, at Thundersplace overall the 3" flaccids averaged out to 5.75" erect (grew 2.75") and the 5" flaccids averaged out to 7.25" erect (grew 2.25").
So the two sites datas match up fairly well.
http://www.erectionphotos.com/softH...dGalleryP01.htm
Flaccid Length--------Averaged Erect Length----------------Amount of increase in inches
1.00-1.49---------------------4.71-------------------------------------+3.50
1.5-1.99----------------------4.89-------------------------------------+3.14
2.00-2.49---------------------4.88-------------------------------------+2.63
2.50-2.99---------------------4.92-------------------------------------+2.17
3.00-3.49---------------------6.07-------------------------------------+2.82
3.50-3.99---------------------6.08-------------------------------------+2.33
4.00-4.49---------------------6.69-------------------------------------+2.44
4.50-4.99---------------------7.00-------------------------------------+2.25
5.00-5.49---------------------7.35-------------------------------------+2.10
5.50-5.99---------------------7.80-------------------------------------+2.05
6.00-6.49---------------------na----------------------------------------na
6.50-6.99---------------------9.00-------------------------------------+2.25
The numbers are somewhat unstable, which more data in each range would likely smooth out. The average increase is 2.5 inches in the chart above. The data above clearly shows that the belief shorter flaccids/growers are just as big as longer flaccids/showers when erect is a myth even though smaller flaccids tend to grow more (giving the myth some validity).