Forgive the newbie, but a quick question from a curious mind..
(And yes, I know we should ask the optometrist, and will!)

But in the meantime, how exactly does the Cyl measurement in a vision
prescription work, in a relative sense?

That probably doesn't make sense, so allow me to explain what I mean.
My MSO has the following script -
(Eye, Spher, Cyl, Axis)
R -2.00,  0.00, 0
L -1.75, -0.50, 30

OK so far, and I have a vague understanding of what the Cyl and Axis
numbers mean.  These numbers are the same as her previous glasses, so
she knows what they are like.

But here's the tricky bit.  Her eyes vary somewhat during the day, and
she finds the full prescription is just too strong and would rather have
slightly weaker glasses.  (She's 50-ish and does not want to go the
bifocal route yet.)  The test was done late afternoon, so given all that
the optometrist let her try lower corrections, and agreed that it would
probably be ok to come down by, say a half-diopter.  But she forgot to
ask what that actually means in regard to the script - would a reduction
only affect the Spherical figure - would it be best to leave the Cyl
figure as it stands, or should it be reduced proportionally?  Eg would
her new Left script be:

L -1.25, -0.50, 30 (ie Cyl unchanged)

or would this be closer to the mark:

L -1.25, -0.25, 30 (ie Cyl reduced a bit also)

I'm guessing the latter..?