Might be an idea to start by winding out the regular velocity adjustment till it gives a low velocity, IE, will be under less tension...then start sweet spotting the inline reg, then go back and set your final velocity at the back...as Parksy said, this is where you should adjust your velocity, once you have sweetspotted your Black Magic.
Alternatively, you could remove the blanking plug, that might be lurking in thebottom of your front block, bung a suitable guage in, sweetspot it, then put the grub screw back in.