Are you COMPLETELY removing the tandem breakers AND the wiring to them? If so, and it was because they were not being used then they were not a factor in the original panel's load, and you are adding 25-30 amps to the panel.
Turn everything on that you can, then use a clamp on meter at the input of the wires to the box. That will tell your total load.
Be careful playing with electricity.
Theory only works perfect in a vacuum.
Cyber Security Protection for Windows C:\ > WWW.WinForce.Net
The new subs [as in submarine] dont have true periscopes anymore. just a video screen feed