Spring force actual rod reading at 5mm from each end of stroke.

f1=extension force piston rod extended
f2=extension force piston rod retracted
f3=compression force piston rod extended
f4=compression force piston rod retracted
fr=friction force=f3-f1,f4-f2
spring rate(x)=f2/f1

the value f1 measured at 20℃±3℃ with the piston rod shwing down wards.