Reduce by contact lenses prescription

Let’s say we have a glasses prescription:
SPH -9 CYL -2.75

That matches real refraction (or contact lenses prescription):
SPH -8 CYL -2

When we reduce cylinder by 0.5, we get only 0.25 real reduction (1):
SPH -9 CYL -2.25 -> SPH -8 CYL -1.75

To reduce real 0.5 cylinder, we should reduce 0.75 cyl from glasses prescription:
SPH -9 CYL -2 -> SPH -8 CYL -1.5

So, should I stick with (1) or make (2)?