This is a problem that can be solved through the power of math.
Let's assume you're using a displacement cylinder cause those are easier to work with. You can make em by subdividing a group of three like this.
Now each of the four faces is 80u. That makes the entire length around it 320u. This texture fits comfortably on a 128u surface, so let's scale up 256u to fit 320u.
320 / 256 * 0.25=0.3125
Plug it into the X scale (cause that's horizontal), wrap it around using alt+right click on the face edit sheet, and viola! The texture is seamless.
Bonus: if the size of the brush is something like 192x192 or 128x128, you won't need to do any scaling.
Or if you're dedicated to using an actual cylinder (this one is 64x64), you have a few more steps.
To find the length of any side, you have to find the length of all angled surfaces. You can use the Pythagorean theorem for that. For example, sqrt(4^2 + 16^2) = 16.4924225025. The total ends up as 199.821631014. Scale that: 199.821631014 / 256 * 0.25 = 0.19513831153. Align your starting face to face then wrap it around.
Pay attention in algebra, kids.