Many thanks 'openBrain' for taking the time to read, react and rectify my post. I had intended to attach the PDF of the part drawing of the Steam dome which I have now included in the event anyone is interested. In truth I had many problems trying to download Graphviz but have now managed to install...
I have found several instances where a constraint cannot be placed or changed unless another local constraint is removed, not sure if they have to be in a specified order to be compatible with each other.
Don't know if this helps, I had a little play with your sketch and found the same issues you have, however if you remove the parallel constraint, change the angle then reapply the parallel constraint it works.
Regret, not able to answer kisoire questions, The space between Radius dimension and line is where techdraws puts it, as I indicated I am new to TechDraw, however it seems that Drawing Dimensions in earlier FC versions was more flexible, I still welcome any further guidance here.