Current Density Value Suitable for Different 3 Phase Transformers

Suitable current density is the density at which the transformer does not overheat, the transformer meets the specified loss, the winding does not fail during a short circuit and the cost is a minimum.

I used to use 1000 circular mill/amp (very conserative) at the start of a design and then change as required to meet the above requirements.

Why are you interested in the current density? (Just curious)

Thanks, I am to design a three phase 200kva 33/0.415kv transformer from cutting to length and arrangement of CRGO steel to tanking-up the transformer. So, this is part of what I need to get the work done. I won't mind if I can get more tips to go about it.

To design this type transformer you should have some standards.
For instance transformer manufactures stock certain size steel. If transformer manufacture stocks 9 3/4 in wide steel and you design a transformer with 10 inch wide steel, there will be a problem.
Also the transformer manufacture should have standards for the distance from the end of the winding to the end steel for all insulation classes.
Also usually the tank is designed by a mechanical engineer using the transformer manufacture's standards.
There should be a senior transformer designer available that can answer your questions more correctly than engineers on this site.

The easiest way to design this transformer is to get the design for a similar transformer and make changes. For instance if there is a design for a 200kva 15/0.415 use it as a basis to start your design.

Designing this type transformer without standards is impractical.

Two good reference books are
Transformer Engineering by Blume et al
Transformers for the Electric Power Industry by Richard L. Bean et al