Maths

Foot: 7.5" around
Pattern size: Medium
Pattern needle: 2.25mm
Pattern gauge: 8.25 st/in

Jen's needle: 2.0mm
Jen's gauge: 7 st/in

64 sts around / 7 = 9.125"
64 sts around / 8.25 = 7.75"

Remeasure foot: 9.5" / 8.5" = 90%
(or 60 sts)

(My sock is going to be about 1/2" too big)

Onward!