diy adjustable struts.advice please

[irsbuggy]

i want to make my own struts for my 1303 beetle(3 bolt struts)

i have strut inserts from a astra gte mk2 (390mm long bottom to top of threaded part).
i plan on using 50mm diameter 2mm walled tube,weld on threaded sleeve(from demon tweeks) and 2 1/4 springs..
questions...
what length should 50mm tube be?
position of weld on threaded sleeve?
length and poundage of springs?
any other advice?