My chandler sold me a laser button that with a bit of minor filing, fit into the slot just fine. I used a 3:1 vang that I rigged up, but the fiddle block was too close to the deck so I extended it with a ss 1x19 wire rope 1/8" diameter (overkill) to get the fiddle block off the deck.
To your original question, mine was a fairly shallow angle as well as my mast had been inadvertently shortened by the PO during a repair.
The stresses on this size of rig are not so great that the angle needs to be exact, and as far as I know the angle of the vang is not regulated by the class rules.
I can sent you pictures from my book if you wish. E-mail me at firstname.lastname@example.org