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.
Don't pay someone to do a job fast, when you can do a half-fast job yourself.
The new subs [as in submarine] dont have true periscopes anymore. just a video screen feed