I would say just eyeball it,unlessyour showing it,no one is gonna be able to tell.
I eyeballed mine and measured somewhat,mine came out nice.
As for splicing,it was just a trial/error thing for me to find the wire that controls it when you turn as opposed to having it on all the time.I mean there are only 3 wires going into your corner light,one being a ground so you have a 50/50 shot of getting the right wire the first time.