BCD to binary conversion

Finally found it here http://forums.terraria.org/index.php?threads/project-bcd-to-binary-converter.30458/

"The double-dabble mechanism is a well known algorithm to convert binary-to-BCD, so reversing the algorithm allows for the reverse conversion. In the reverse algorithm, the information stored in the bits is right shifted, and then any number greater than 7 stored in any of the BCD units is reduced by 3". Simple and clear.

Was worth of an hour of searching as there were some more sources which are badly explained or just seem to be wrong!