Two ways to go:
1. Do the demand calculations according to code. You may not be able to add load based on these calculations.
2. If it can be shown that the existing load, etc.
There are two ways to show to the AHJ that the existing load is low enough that more load may be added.
a} Recording demand meters on the existing loads for a time acceptable to the AHJ.
This may be 6 to 12 months.
b) If this is a main service transformer, ask the accounts payable department for copies of the last two years of power bills.
Look at the maximum demand each month for the last two years. The AHJ may allow you to ignore the highest demand as an outlier, it depends.
Note: when adding KVAs, PF matters. If the added load is at a significantly different power factor than the average PF of the existing loads, the actual combined KVA may be less than the simple sum of the existing and added KVAs.
That said, simple addition is always safe.
Don't worry about the PFs.
But, If the measured KVA after the addition of new loads is slightly less than the calculated KVA, this is probably why.
--------------------
Ohm's law
Not just a good idea;
It's the LAW!