Ok, so there are many opinions on this, but I will be listing the most cost-efficient ratios.
So for shields per reactor, if you want to have fast reloading of your shields do 1 reactor per 2 shields. But if you need more shields for less money you can do 1 reactor for 3 shields.
For factories per (large) cannons 1:1 is usually enough.
Any more and it becomes too explosive or taking up more money than it's worth.
For standard, it is 1:2.
Also, for railguns 1 reactor can power 10-12 accelerators If they have an efficient way of getting to the outer ones.