Example:

Step 1

You have a bullet with known G7 BC of 0,261. It's 0.264" and 123 grains. Now you can calculate form factor (i7) for it. i7 = weight/7000/(caliber^2*BC) --> i7 = 123/7000 / (0.264^2 * 0,261) = 0,966. This means that bullet's drag is 96,6 % of G7 projectile drag.

Step 2

You can use G7 drag data table to find out Cd for velocity in question (for G7 projectile). See picture below how to do it using G7 graph.

Or using robert's BFx function in excel =BfX_Cd(900;"G7") this gives you 0,262 as a Cd.

Step 3

Since you know that for your bullet drag is 96,6 % of G7 projectile you can calculate for the bullet in question that CD(900m/s) = 0,966* 0,262 = 0,253

Clear enough?

Now if you are measuring some odd shape bullet that doesn't fit very well to any know drag data (G1, G5, G6, G7 etc..) you can start to work with the closest one and using your measured velocities find out where it goes wrong. Then just do your own custom drag table to modify it as necessary. This is possible since Robert kindly added this option as well. You could use some "best fit" tool to do this but good old trial and error works as well.