CR2450 Battery at 2.95 Volts
For CR2450 with a capacity of  >600mAh at a 0.4mA rate;

If ESR is &lt; 20 Ohms then you have at least 50% capacity left.
If ESR is > 50 Ohms then there is not much life left (20%?)

Use a 1K load and measure voltage before after and compute voltage drop in % and multiply percent of 1K as your ESR. 


Capacity drops with a load &lt; 100*ESR and rapidly with &lt; 50*ESR
Capacity increases with load resistance up to ~1000* ESR


Based on Voc=2.95V or -0.05V from 3.0V, I estimate it is at 90% SoC

Using a Nickel spacer will work but obviously it will not last as long as a thicker CR2477 Lithium cell with up 1000mAh.