Your methodology seems correct. Just take your time and make sure it's right. Once you have it right or the best you are capable it might be worth taking it to a machine shop and let them double check, that is if you have a reliable shop you can use.
Have you called Z1 or SZ to get their recommendation? Either company has probably dealt with this on more than one occasion.
Once they are in the correct location Mr Obvious says to mark them. Good luck and keep us updated with your progress.