Flat Duct Bend Dynamic Block Scale Problem

[jamie_lovelock]

I'm trying to create a dynamic block which is a flat duct bend. I almost had no problems creating the block except the outside line. 

The outside line needs to be 20mm away from the bend itself (which represents insulation) no matter what I do, I cannot figure out how to scale it but also keep it 20mm away from the bend. I originally thought that I could do it with an angular dimension so as I stretch or scale the block the dimension would follow it. It needs to stretch/scale from the center of the block outwards

I've tried using constraints but I've found they are incredibly buggy. Does anyone have any ideas on how to do this?

Thanks

[h_s_walker]

Check my reply in the link below

Re: Any way to keep linework a fixed distance apart? - Autodesk Community - AutoCAD

[jamie_lovelock]

It was really that simple!

Thanks a lot for your input!