Reduce CYL by percentages or by diopters?

When we reduce SPH power for new normalized or differentials, we do it by decreasing the diopter power by 0.25 in most cases. However, how should we approach reducing CYL power? Should we also do it by decreasing CYL power in dipoters equaly in both eyes? Look at this example:

Here is my case, without asking for dipoter specific advice (all values are negative CYL power. For simplicity I just typed the numbers without a positive or a negative):

Doctor’s CYL Prescription: 2.5L, 1.5R
Current Normalized (substracted -0.75): 1.75L, 0.75R

As you see I simply dropped 0.75 from the doctor’s prescription. However, look at the percentage of prescription kept in each eye:

Percentage of Prescription kept: 70%L, 50%R

As you can see, this reduction is much greater for the right eye, even though the substracted power in dipoters was the same. If I was to keep 70% of CYL power in both eyes it would look like this:

Keeping 70% CYL on both eyes: 1.75L 1.05R

Because Astigmatism is directional blur, and tends to be more complex than simple SPH blur. Would it make sense to approach CYL reductions with percentages?

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This is a tough one, but basically @jakey recommends just going with straight diopter drops rather than by percentages like that. I’ve noticed the mathematical discrepancy there and also with the “ratio” between-eyes as comparing diffs vs. norms.

Too much of an |absolute| change, I think, is the issue.

It might be interesting for you to try it that way and see how it works out. I’m basically counting each 0.50 change in cylinder like a 0.25 change in spherical equivalent, with the exception of one round in which i dropped both sphere and cylinder 0.25, giving me a 0.375 overall spherical equivalent delta. After that round I decided to separate my spherical and cylindrical drops.

I also sort of broke out of the formal EM method while equalizing and while gettng some initial astigmatism gains, but I was very high (-3.25 -3.25 x OD, -2.75 -2.75 x OS) and it was pathological.

Let us know what you decide to do and how it works out.

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You craft your close up glasses by comfort. You don’t want the astigmatism to be noticeable in the diff and many times you can reduce some of it and even all of it without seeing the astigmatism up close