And the reason that a plain ordinary tape measure from Sears won't work for
this purpose is -- ?

While we are on the subject, I have a question about measuring
"tricks" of the trade.

I am installing an old (single-panel tilt-out/kick-out) garage door in
a barn I built - taking it off the house garage and installing it in
the barn (replacing the one at the house with a new four-panel door).

The door is currently installed in a garage with a ceiling and te read
supports are hung from that.

The barn has no ceiling, just the rafters exposed with two pitches
(front part of roof is eight feet or so and the rear section about
tweleve, so the pitch at front is steeper than at teh rear and the
door spans the two (with the ridge beam off center of te door
opening).

So, I have to place the hangers and fasten them to the rafters which
are at different heights (where I need connect them to the rafters)
relative to the concrete slab below. Essentially, I need to find a
point "in space" where the hangers will meet the rear of teh
horizontal supports/wheel chase(s) the top of the door rides in when
opened.

I don't have a helper to hold one end of a tape. I have made a story
board marked to the distance the old hangers are from the existing
installation - two-by set perpendicular to the floor on a rudimentary
"stand" and marked at the 80" point as a reference.

I've placed a couple of two by six boards between the two rafters in
te approximate location to hold the angle "iron" from which the
support arms will "dangle" and leveled those surfaces so the "angle
iron will lay square relative to space and the flooring.

Thee are no interior walls - just the exposed studs at this point so
the laser levels I have light up a piece of framing here and there but
do not provide a nice clean line to followand I don't trust them all
that much when I can see them.

The door is about nine feet wide by seven feet tall.

I need to establish the mounting points before I remove the door so as
to accomplish the R&R and swap in one day lest we finally get some
heavy rains while the doors are down and the garage & barn open to the
elements.

As indicaed, I am moving along, but thought to ask here for any tips
and techniques for that second measurement before committing. I have a
six-foot level (and shorter levels as well, carpenter's square(s) ad
tape measures, plumb bob(s) and line levels. Also have a B&D laser
level thing and another that does a right angle level/plumb (one beam
out the right side, the other out the top side) and can be locked to
"shoot" a fixed right angle for "layout."

Would appreciate a Reply to Sender if you have a technique I might
better employ.

Thanks.