Here's a quick run down on properly determining pushrod length that I hope is helpful. This is also a follow up on the Patriot 320's and some of their issues.
IMO valve train geometry is way overlooked by even some of the most professional builders, and there's some real misinformation regarding determining proper pushrod length and it's relation to rocker geometry.
The only way to determine proper pushrod length is to check your rocker and it's relationship to the valve at mid lift. Period. This can be done during mock up, but better done after complete assembly with the full valve train. Pushrods should be the last thing you order for an engine since there are so many variables that effect pushrod length. You want to be at a point in your build where you're not going to be changing anything like deck height, head surface, valve depth, or even your cam. Now I realize that some of this may be splitting hairs to some, and like everything else it needs to be kept in context, but this is what's involved.
First, set up a dial indicator on the cyl head so you can measure actual lift at the valve. Be sure your indicator is on the same angles as (parallel to) the valve, or your readings will be off.
Install whatever pushrod you're using or checking, or your adjustable pushrod set to as close as you can to whatever length you think you'll need, set your lash accordingly and then rotate the engine and measure total lift. Divide your actual total lift by 2, then rotate the engine again until you're at that half-lift number (called mid-lift). What you want, ideally is, for the rocker to be at 90* to the valve stem at mid lift. By the rocker, I mean the line that represents the center of the rocker, from the center of the trunion, through the center of the roller tip.
If your pushrod is the right length, this is what you'll see.
This is NOT what you want to see:
This is the Patriot head using a 9.400 ex pushrod (.100 longer than they recommend) with a new Isky 1.7 BB Chev rocker. Using the retainer as a reference to 90* to the valve, you can see the line through the rocker's center lines is no where near 90* to the valve stem. The pushrod is way too short, and only netted a valve lift of .603". With a lobe of .390, and a rocker ratio of 1.7, less .025 lash, we should be somewhere near .638, less cam deflection.
I determined that the patriot head needed an ex pushrod length of 9.77" - 9.78". Patriot recommends 9.35" for their exhaust. I also believe I'm using an ex valve .100 shorter than what they recommend. With their longer ex valve, the rocker would have to sit that much higher and the problem would be even worse. Here's the Patriot head with the longer (correct length) pushrod. Again using the bottom of the retainer as a reference to 90* to the valve, you can sse this is very close. There was so much added lift gained by the proper length pushrpod that these actually turned out to be slightly long (by about .020")
This same rocker, same lash, with this pushrod netted a lift of .638".