I tried the formula for the C/2014 Q2 Lovejoy comet.
My equipment is a Canon 500D (pixel size = 4.7 µm) with a Tamron 135 mm F/2.8 lens open at F/4. The comet, yesterday was at 13° declination.
The formula (in blue below) gives :
t = 25x(4.7+4)/[135xcos(13°)]=1.7s rounded to 2s
The stacking of 67 photos with 2 s exposures gives the following result (not enough to correctly see the comet but enough to imagine it) :
http://www.sahavre.fr/forum/download...=949&mode=view
The stars are round !