You can finesse the value you get a bit by playing around with how the upgrades are arranged btw. Checking the result against the stats display on the right (meagre as they may be) as you go. There are rules for how the orientation of upgrades to the main part affect the boost you get from them, apparently, but I just go by feel.
Just know that once they light up, that's usually not the maximum boost you can get yet. The difference can be quite extreme. Switch two around, hey presto, 10% better. Sometimes its better to have the main part on a super slot, and sometimes it's better to have one of the upgrades there rather.