I don't have a Rostock style but here is what I do.
Go into firmware and switch it from delta mode to regular. (If you don't know how to do this look up a tutorial on uploading firmware.)
Move an axis 200mm. Measure the actual travel. Say you are off and actually move 210mm. Multiple the steps/mm in eeprom (you can do this in repetier host) by 200/210. Repeat for each axis and do it a few times to make sure. Switch back to delta mode and cross your fingera.
Note: There are many things that need to be calibrated like this so look through the eeprom values and see what you need to set. Don't expect anythong to be right.
Happy printing!