One additional area of concern is Filter Factoring, how much additional exposure to use. It is my understanding that when stacking filters you do not combine the factors, a 1.5X and a 2X do not make a 3.5X. Use only the single largest Factor for the stack. I don't think the order is important.
Just thinking ahead.
It always amuses me that, even with photographers who are thoroughly versed in the Zone System and have carefully tested to determine their personal Exposure Index, people blindly apply a filter factor despite having everything they need to determine the correct exposure compensation. What I mean is that a manufacturer will provide a filter factor based on an average effect of an average filter on an average scene with lighting that corresponds to a few hours before and after Noon.
To ascertain the correct exposure compensation for the filter that you have chosen to use for the scene before you at the time of day that you want to make your photograph is actually very simple:
Hold the filter in one hand and with the other hand point an averaging meter (such as a Weston) at the scene that you wish to photograph and note where the needle is on the scale (or exposure if using a meter such as a Sekonic that does not use a scale). Keep the meter pointed at the scene and then place the filter in front of the selenium cell/sensor and note the effect (i.e how far the needle goes down / how much the exposure is effected). You now know that for this particular scene at this time of day with this particular filter how much exposure compensation you will require. You can then proceed to meter the scene how you normally would and then apply the filter correction that you have determined. Using this method you will often find that a dark red filter may require anything between 2 to 4 stops of correction - an important variation if shadow detail is important to you.
PLEASE NOTE: The above method does not work with CDS meters as they have a very skewed spectral sensitivity.