I would add that since declination is the angle of a star north or south of the equator, then at the latitude you will be at and photographing a southern horizon, it is quite possible that you will be photographing stars with zero declination.
This is not the same as your declination which is your latitude and from my understanding, it is a combination of your latitude and the star declination which determines how quickly a single star in the sky moves. But you are not photographing a single star. You are photographing a large chunk of the sky and each star is moving across your field of view at a different speed. So if you want astronomical accurracy, you must first work out which star in the sky is moving fastest. i.e. the one nearest to zero declination. But then you have to factor in your latitude which none of the examples given have mentioned.

Hence the formula T = 343 / (F Cos d) is a fudge formula and assumes 35mm film format. Assuming 0 declination and 50mm lens, then that gives 6.86 secs. Thats for film. But use a 50mm medium format lens, then things change depending on the lens design. i.e. how much of the field of view is projected onto the useable camera sensor. So you must know what angle of view is projected onto the sensor area to make an accurate calculation. It all becomes a royal pain for the casual night sky shot. So unless you are serious about finding the correct star to work from, its declination, your exact latitude, the required level of accuracy for high density sensors, and doing all the calculations based on the actual field of view on your sensor, then a simple worst case scenario makes life very easy. Or the fudge formula adjusted to your field of view on the sensor but you will require trial and error to work that out.

Having said all that, since your camera is digital, you should have zoom software in the back to see the images in fine detail. Therefore why can't you just take some shots and check the trails or not at the time, so you can be 100% sure you have no star trails. Thereby negating the need for any of this astronomical stuff.