You can check the maximum firing range of the ships in the text data files... not sure if it can be checked in-game. As a general rule, you should EITHER add Fire Control attachments to the Light Cruisers but not to the Heavy Cruisers (thus extending the screening ships' range out past the capital ships' range), or you should build Light Cruisers that are at least two generations newer than the Heavy Cruisers (eg: CA-I plus CL-III or -IV, or CA-II plus CL-IV or -V). At least, that's how it worked in the un-modded base game.