"Naio".....Unless you're an engineer, it will be tough to calculate what you need. I added struts to the front hood of my Dutch Star. It originally came with one of those hood rods like you see in cars.
Figuring the position isn't too hard, but depending on where you lift figuring the poundage becomes an issue. Luckily on my project, I had the same length pistons on my rear hood. I removed those and tried them. They just didn't quite lift the hood and hold it. They were 25# models. I ordered two 30# models and they worked perfectly. This isn't scientific, but more of the poor man's way.