EDIT:My intention was to destructively modify petri lens for use on EF DSLR.
I found some good Petri lens selling for less than 100RMB on the local auction site. I searched and found that petri breechlock is only 0.5mm shorter in flange than Canon EF so I would like to convert them. But after searching a lot I still don't know how exactly is the the petri mount's flange calculated. Would be gratified if somebody can tell me from where it is calculated.