Please see below ( I want to do step 3 - change a value by 90 degrees)
but the answer has to be between 0 and 180

e.g. 45 becomes 90

e.g. 95 becomes 185 (but this is wrong, so SUBTRACT 90 instead) anwer
is 5



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from http://www.usaeyes.org/faq/subjects/script.htm

Minus Cylinder or Plus Cylinder

Spectacle prescriptions can be written in two value sets, minus
cylinder or plus cylinder, which are mutually exclusive of each other
but provide the same information. As a rule, ophthalmologists write
scripts in minus cylinder whereas optometrists write scripts in plus
cylinder. Why these two professions cannot get together and decide on a
common method of reporting refractive error is impossible to explain,
but if you have ever had competing siblings in your family, you may get
an idea why this has not yet occurred.

To convert a minus cylinder form prescription into plus cylinder, or to
convert the plus cylinder form into minus cylinder, do the following:

1) Add the sphere and cylinder powers together; this becomes the new
sphere power.
2) Change the sign of the cylinder power, from minus (-) to plus (+)
or from plus (+) to minus (-).
3) Change the axis value by 90°, remembering that the axis must be a
number from 1 to 180.
The following lens prescriptions, therefore, are equivalent and
interchangeable

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