It's ok how is it.
No need to change any thing.
As for your maths, no idea what you caculated, but in order to do that 19 K damage, you need 4 cerberus guns, not 3.
So if a core 1 base have all 3 kinds of commidies and a single BS with 4 Cerbs firing each cerb 16 times per 16 seconds.....by having 7 BS with the status i mentioned, it will take 98 minutes to destroy the base and yes if it have shields.
Multiply the numbers by two if the base core is 2 and so on.