Okay this may be a bit OCD.
How do you make a perfect isometric cylinder like this?
Connecting the two circles has always been eyeballing for me. How do you make it perfect?
One option is to use Illustrator's 3D Extrude & Bevel. Start with a flat circle and go to Effect → 3D → Extrude & Bevel...
There are isometric presets already available.
Another option is to use an isometric grid. There is a nice tutorial on creating an isometric grid in Illustrator here: How to Create an Isometric Grid in Less than 2 Minutes!.
I draw stuff like that all the time. Usually I use the Common Tangents script by Hiroyuki Sato. It works quite well and helps you in any other find the tangent situation too. I find the script indispensable.