Good points. Thanks, I'll try that.You only get a set amount of SP for merging a unit in. You don't get much for giving a 3* Olivia to a 5*. However, as I said in my previous post, you can get around this by using the system where merges bring over all their learned and unlearned skills. So you level up, inherit a few skills, learn them and THEN merge.
While merging I did notice that the set amount of SP you get for merging in seems to increase the larger the merge ball gets. The +3 gave I think 240 SP when merged into the +0. I wasn't paying full attention so maybe someone knows more about this.
The problem here of course is that leveling and farming SP on Olivia fucking sucks which is why I did all that in the first place. Even if I end up just promoting to 5*, creating the merged 4* got me so much SP that it was worth it.