Maybe about 600 ft if you have enough shock, I dont know what the wheel rate ratio is the for rear of the Mk 4. The Mk 2 and 3 is almost 100% and I use about 550 rear. I would guess about 600 for starters in the back.
I would think that these cars will wind up at about 650 ft and 750 rear wheel rates. Better off a littl low than too high for starters tho,IMHO.
That discounts the sway bars, I use the stock rear bar and no front on a welded car, and a small ajustable rear ,on the Quaife cars.
You can figure the wheel rate to spring rate ratio by measuring the leverage of the wheel to the spring. IE; If the wheel moves6 in , and the spring connection moves 3in , you have 50%. To get a wheel rate of 500#, you would need a spring of 1000#,etc.
If the front strut points at the center of the tire contact patch, it is about 100% , IE Mk 3 Golf. The Mk 2 is about 96%, The MK 1 is about 90%(est).
The difference is the strut angle.