I use the same one as ovaz...
http://www.americanpregnancy.org/duringpregnancy/hcglevels.html
This is the website I use :)
scroll down to the bottom of this page, and on the lower right side underneath related tags, there is a most viewed health pages.. its under the early ultrasound/hcg/progesterone guide, and i am not sure where the other one is!! sorry i couldnt help with the website but i thought that might help a little bit