It sounds like you are looking for image jump since you mentioned displacement, but I am not sure.
You need to find out where the OC of the segment is.
You said that the width is 40mm and the depth 25mm.
So your seg
oc is 5mm
To find image jump you use Prentice's Rule using the 5 mm times the add power.
For example a +2.00 Add with this seg would produce 1^BD of image jump.
Now you also mentioned the reading level and how far above the seg your OC is.
Now the top of your seg is 4mm below the OC, your reading level is 12 mm and you now know that the seg
oc is 5mm below the top of the seg.
(5 + 4) - 12 = -3
The negative indicates that you are going lower than the seg
oc.
If you have a -3.00 X 90 with a +2.00 Add
Calculate prism at distance and near
P
distance = 3.00 X 1.2
P
distance = 3.6^BD
P
near = 0.3 X 2 (I use 3mm because the difference between the seg
oc and how far below this the readers eye is)
P
near = 0.6^BU (BU because it is below the seg
oc)
So you have a total of 3.0^BD at reading level.
I attached an image to show the relation between the seg
oc, distance OC and where the reader is reading.
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