Determining stability as a function of center of mass

[Sinsation]

Hello,

I have a question regarding stability and how to design for a stable rocket.
I have an initial center of mass for the full rocket without fuel/propellant, and one for a fully loaded rocket with propellant.
As the mass changes, and due to the placement of the tank, the center of mass for a fully loaded rocket is moved quite substantially to the front of the rocket.
But as the rocket is launched and the fuel is being burnt, the total weight of the rocket decreases, and I am assuming the center of mass then will move back to where it is for an empty tank.
With the current calculation, over half of the ascent occurs with an empty tank, and this leads to my question:

When designing the fins for stability, should one base the calculations on the center of mass for the fully loaded rocket or the empty tank?
With a fully loaded rocket, the stability is over 6.0, while for an empty tank, the number is around 1.5-2.

Hope someone may help

 

[Azamiryou]

Design for the worst condition throughout the flight.

Solid propellant rockets usually have the fuel well behind CG, so the rocket gets more stable as it gets consumed and you use fully loaded weight to check stability.

If your fuel is stored ahead of CG or anything else makes the flight less stable (say, a booster stage takes its big fins with it) over time, figure out what the least stable point is and make sure it's stable then.