I'll chime in if no one else is, but as a disclaimer: This is from memory of the IH 1300 mower, which is the same as the 505 (Ford rebadged the IH 1300 as the 505). I could be misremembering or getting some aspects mixed up with the Deere 350 mower, which has a very similar linkage (but using a chain instead of a cable). Not many people are familiar with the Ford 505 version, so you might have better luck re-posting this with "IH 1300" mower in the title, because the 1300 was extremely popular, and lots of folks on here probably have used or are using one.
The mower, when running on level ground, should be set such that when the hitch arms are dropped just enough so the inner shoe has also just started resting on the ground, which will be a function of the cable length. You don't want that cable doing any lifting at this point - it should be just slightly slack. The purpose of the cable is so when you lift the hitch, the cable gets pulled and by the geometry/linkages the cable's connected to, the end of the bar raises at a greater rate than the main frame.
If you're trying to use the cable to set the maximum incline angle, it might be a little difficult: The more you raise the hitch, the steeper the angle will be. Usually you wouldn't worry about this as you typically intend to be running with the bar resting on the shoes on the ground, or with the hitch raised only partly (say, at the end of a run when you're turning to enter the next swath). You're kind of expected to know enough not to lift the hitch too much and make the bar incline too steep. You might have to play with cable length if you only want it to raise 30 degree inclination when you raise your hitch all the way, but I think you'll find it won't really work that well. If I remember right, the cable usually causes it to raise far more than 30 degrees when the hitch is lifted to its highest point, which lets you pass through gates and narrower openings when raise it. If you set it so you only get 30 degrees at max hitch height, I'll bet the cable will go slack long before the bar gets back to level, causing the far end of the bar to drop to the ground before the near and - an irksome condition when trying to cut a field as it's more likely to plug as you try and raise at the end of a run.
I think you're better off setting the cable length as per above (just slightly slack when dropped so the inner shoe just touches the ground) and just be careful not to raise the hitch uber-high when running. Of course, how high your three point lifts will make a difference on how much the bar inclines when you raise the hitch all the way. On our Deere 350 mower with a similar linkage, when hooked to our Deere 2120 which can raise the hitch arms uncommonly high, the bar goes almost vertical at max height. Hook the same mower up to our Fordson Major and which has had the three point arms modified to drop extremely low for logging and only raise about 24" max, and it the end of the bar barely lifts higher than the frame. And that's with the chain (cable) length set properly as per above in both cases.
As far as decline angle, if I recall correctly that's just whatever the ground is - you just have to know enough not to run it on ground sloping away too sharply from the tractor. I suspect the 1300 has a stop to prevent it from declining too far, but I can't remember for sure.
