My method has been to go into a station with
Outfitting and then select your module and check your current modifications. Now open up a web browser and use Inara
(inara.cz) to took at what the new system will do for you.
For the FSD for example go to
(https://inara.cz/galaxy-blueprint/2/) and scroll down to the G5 and you will see the max 100% completion of G5 you will get, then look under for your Mass Manager or Deep Charge, depending on your FSD size and then you can compare yours with that and see which is better.
I have been doing this now with all my ships and modules and it makes it so much easier to determine what is worth it or not. I do have a spreadsheet with screen shots of all my pre 3.0 engineered modules though by ship, so it's easier to look at that for me, but most people didn't do that so
Outfitting will be the best way, unless you added all your ships details into
Coriolis or something...