844 nm is the still-air range, in nautical air miles. You have not considered the headwind. Note that at this altitude and power setting the TAS is 160 kts, groundspeed with a 40-kt headwind is therefore 120 kts. Use the equation on page 40 of the CAP, in effect
nautical ground miles = nautical air miles x G/S ÷ TAS
in this case ngm = 844 x 120 ÷ 160 = 633 ngm, the range over the ground.
Hope this helps
Send Clowns
Flight planning instructor, BCFT