From someone that makes the frazzlin' things here is what I have migrated to for many years. It is bits & pieces from some of the previous posts in this thread.
From many years ago when I was shooting bench rest rifles, I pilfered the sandbag that was made with a cradle that held the bottom of the rifle stock and there is where I place the butt of the rod and the other end in the rollers I use. I also am working under an 8 foot light bank of the "environmentally safe pigtail flourescents" that cast a perfect straight line down anything that is round under it. Then I eyeball it.....the sandbag keeps the rod from turning.
On the decals I supply, you can also scribe a straight line down the top of the transfer film with a fine point permanent marker to act as a guide line to place directly over the reflection line.....it's a bit tricky as you will partially block the "light guide line" as you align the marker guide line....but at least you will be able to see if any thing moves.
However, first and foremost......make sure the rod is in the correct orientation BEFORE you start aligning the guide lines to apply the decal. because if it isn't, the decal will be perfectly aligned to the rod blank......just off center.....trust me...been there!!!!
Thanks for reading this. It may help a wee bit.
Sweetwater Fly Rods & Golden Touch Decals
"I'll create your custom rod decals so you won't have to"
"The best decals are the ones you STICK with....."