If you are measuring up a custom coilover kit for your car, this suspension calculator might help you. Enter some key dimensions from your car’s suspension and the damper calculator/strut calculator will give some key dimensions (body length, spring length, stroke etc) which can be used as a guide ONLY when choosing your custom coilover damper/strut. Spax for example sell custom coilovers where you can choose mounting types, body and rod length.
I found this article which has excellent information on the subject of rebound:bump ratios of dampers and their effect. Article is concerned with motorbikes but I think it carries to cars.
On the subject of helper and tender springs. Suspension Secrets has a good article here. If you are struggling to remember the difference between helper and tender springs. I say helper springs “help” keep the springs located. (Tender springs are a lower rate spring).