Coil-overs should be mounted so that they are compressed 1/3 of their stroke at ride height. For example, if the shock eyes are 10″ compressed and 13″ fully extended, the stroke would be 3″. The designed ride height of this shock (coil-over) would be 12″

TCI and Heidt’s standard steel rear coil-overs mount with a ride height of 13″
center-to-center. Street rods with solid rear ends (8″ & 9″ Ford for example) normally mount the coil-overs so they are vertical when viewed from the side of the car, and at an angle (closer together at the top) when viewed from the rear. Normally the shock angle will be
between 20 & 30 degrees. With each upper mount 4-1/2″ inboard from the lower mount, the shock will be at a 20 degree angle. This gives a firmer ride. With the upper mounts in 6-1/2″, the shock will be on a 30 degree angle. This will give a softer ride.

For Aldan, and other coil-overs with a 12″ ride height, offset should be 4-1/8″ for 20 degrees and 6″ for 30 degrees.

Ridetech has a lot of great info on their website: coilover info.