Okay, I guess I figured out how to do it:
Similar to positioning a circle on face , draw a line approximately where I want it, then grab the end points and drag them to the sides I want a certain offset from and the dimension dialog box comes up. Works but seems like a lot of steps to get a certain position, esp when trying to reference two different edges. A pop up orthogonal line telling me I'm at 90° or aligned to some nearby endpoint would sure help. Ashlar Vellum calls it a 'Drafting Assistant'.
My preference would be a icon over on the left side that when invoked via a thumb swipe/tap would convert the pencil form regular line to construction line and then a second icon nearby that would let me do an offset to either this construction line or to another edge or single line. Hit first icon, flick pencil over the edge or line I want a construction line to 'spring' from, hit second icon, and use pencil to drag an offset line in desired direction, numeric input screen pops up to input desired offset. Also, ideally, all construction lines would go on their own layer, with toggled visibility .
Pretty fast and simple, would help a lot in layout and positioning