To expand on Mark's comment, you already know your cal course is 304.8m, here is how you calibrate in metric (and this formula will work for any metric cal course):
Calibrate on any METRIC cal course using this formula:
(1000 / cal course length in meters * Avg counts) * 1.001
e.g. for your cal course
(we're going to assume your avg counts were 3434)
(1000 / 304.8 * 3434) * 1.001 = 11277.6706 ct/km
Convert to miles to layout mile splits
( 11277.6706 * 1.609344 = 18149.6515 ct/mi )
Basically if you learn those formulas you can calibrate on any course easily