Hi David,
Sorry, I don't know what happened to the first image (it seems to have shrunk a lot and I'm not aware of anything I did) and I'm at home with the VM at work so I can't reproduce it quickly, although I think you can still read it (let me know if you can't and I'll have another go).
As the smallest unit of measure you deal with is a 2 inch segment I created a Unit of Measure Schedule as 2INCH with FOOT as a second unit of measure as 6 on 2INCH;
I then set the Default Purchasing U of M to FOOT with 2INCH set as Not Available;
In BOM Maintenance I added the TUBE Item which pulls through the UofM Schedule and set a quantity of 1;
Ian