Duration at .050" is the amount of rotation degrees the valve is open starting at .050" after opening to .050" before closing. It's very hard to see on a dial indicator when the valve starts to open from zero lift, it could be 15 degrees or more of rotation before the valve is at .050" open, same with closing. So as an industry standard, the cams are advertised with duration @ .050", it's easy to see on an indicator. The first and last .050" is very hard to measure accurately, it usually takes a dial indicator that reads in 10,000's to get a somewhat accurate reading.
Valve lift is just that, the amount of lift the valve head moves off the seat.
Camshaft lift is how much total lift the lobe has from its base circle.
Rocker arm ratio describes how much more lift the valve has over the camshaft. A typical V8 usually has a 1.5 ratio where the valve moves 1.5 times as much as the lifter does. Race engines increase the ratio up to 1.7, 1.8, 1.9 etc.... you gain lift as the ratio increases. It comes at a cost though, you increase pressure on camshaft, pushrods, and rocker arms as the ratio increases. With the older engines like the 235 that didn't get a lot aftermarket parts, you are stuck with factory spec parts, where as a small block Chevy has everything you can imagine from the aftermarket suppliers. Also, the camshaft bore in the block dictates how much lift a camshaft can have. It must fit in the hole. Higher ratio rocker arms gives more lift over the camshaft phisical limits.
Centerline of a cam lobe is the point dead center of the total duration of the lobe.
Centerline of the camshaft is the centerline between peak opening of intake and exhaust lobes. Two per cylinder, each cylinder being the same.
https://claysmithcams.com/blog/defining-lobe-center-/All these number describe the timing of the valves to the location of the pistons. More lift equals more time for air to enter and exit the cylinder, longer duration means the valves are held open longer for more time to fill and empty the cylinders. Lift and duration do the same thing. The amount of air flow a head can supply is directly related to camshaft choice. If the cylinder does not flow air at high valve lift, there no need for a high rocker ratio. A 235 head does not flow a lot of air, thus the very mild camshafts.
