Sounds like you are having problems getting the 'circle' correct around the lettering?
So you have a white circle, black lettering and then the tube would be a different color?
If so, I would suggest making up a couple stencils.
One would be the circle itself. Circles and ovals are fairly easy to do on a plotter.
Do another stencil of just the lettering.
Go ahead and paint the tube the color you want. Place the oval stencil on there, paint it white.
Then for the lettering, place the stencil within the oval and then spray the lettering black.
You can only do the above as long as you don't have a 'bleed' type paint for the main color of the tube (like a candy type paint)
You can spray the area black. Add a stencil in which the lettering will be covered with the circle exposed.
Then just spray the circle white. Pull off all stencil and it will leave black lettering with a white circle. Then you will need to mask up the complete logo. Then spray the main color.
You could make up an exact stencil of the round area so you do not have to hand tape this up.
Again, if you have a 'bleed' color, this could bleed onto the white circle.
You also have to watch your edges on these as they could be thick on you. You will be painting white over black which will take quite a few coats.
Have you thought of having someone make up some water transfer decals?
These would look more professional and even have the look of 'painted on' since they are so thin. You can add an intricate border around the lettering (like a silver).
Must less hassle than doing all that stencil work. This also will eliminate the bleed problem along with the edge problem mentioned above.
Attached is a job I did about 5 years ago that the customer had a water transfer decal made up. The decal on the sides of the tank.