How Do I Predict Image Scaling With of Photofinisher Prints?

Hello All

I am getting 4" x 6" photo prints (by a photofinisher such as
Walgreens) of some digital files. I want to ensure that a certain
vertical element in the photos is 2" high when printed by the
photofinsher.

The majority of the images are 1600 pixels wide by 1200 pixels wide
with a resolution of 180 pixels per inch.
A few are 2304 pixels wide by 3072 pixels high with a resolution of
180 pixels per nch.

Here is what I did

1. Opened the JPEGs in Photoshop
2. View > Rulers so I have the rulers along the left and top edges
3. Select all
4. Edit > Transform > Scale
5. Scaled the selection so that vertical element in photo is 2" high
according to the Photoshop rulers.
6. Cut the selection from the image file
7. Paste it back into the image file (this centers the selection on
the canvas)
8. I then opened the files in a program called Compupic Pro. It has a
feature that lets you crop photos when printing typical to typical
photo sizes. I cropped to a 4" x 6" print.

i sent a couple of test files over to Walgreens, and when I picked up
the prints, the vertical element was only 1.5" high instead of 2"
high.

How do I scale the photos to successfully control the size of the
final print. Is step 8 the cause of my problems?

Thanks in advance for any ideas on this.