the astigmatism notation must be interpreted by the optometrist.
If you are nearsighted (minus prescription) you probaly will start to notice astigmatism blur before myopia’s blur when you’re puting away an object far from your eyes.
This means if you your axis is 180º, the blur will appear in horizontal way, with sharpness in the vertical way. So, if you see perfectly sharp at 25cm, and at 30 cm you notice a horizontal blur, this means you’re more myopic in the horizontal way. In order to correct this aberration, you need to consider the minus from the myopia combined with the minus from astigmatism.
A plus astigmatism prescription would weaken the minus prescription, but only at the perpendicular edges of the cylinder axis. So you need to put a stronger prescription only at the left and right edges of your sight.
For example (the amount of diopters are slightly random):
image starts to blur at 30cm = -3,00 Spherical
180º sharper line noticed at 25cm (blur kept at left and right edges) = - 0,50 90º . This will need a cylindrical correction, which is -3,50 myopic only at the left and right edges, and still -3,00 vertically.
Most optometrist uses the plus convention for astigmatism, so the CYL would be +0,5 180º, which means the vertical axis will be weaken by the CYL correction, keep the vertical axis proportionally less myopic than the horizontal one.
So if you see -3,0SPH and 180º +0,50CYL at the prescription , the glasses manufacturer understand this as a sharpness on the vertical way and the need of more prescription at the horizontal way, (because the spherical prescription is the stronger one) and actually will manufacturate a -3,50SPH 90º +0,50CYL, keeping the horizontal axis at -3,50 and backing the vertical to -3. It’s the same thing, doesn’t matter.
I’m actually, had this doubt at the zenni optical website, and just ignore my CYL and order an only SPH glasses.
Result: after three weeks, my astigmatism seems to be weaker than before