http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htm

Idling/running vacuum may be a tad lower that the 18-22 stated (but not much) due to engine age and increasing clearances between rings & the like, but vacuum is vacuum and this will give you a good starting place on how to read your gauge and answer your "remains low" question.

Bad timing can cause bad fuel economy, but like someone else in this thread said, a restriction in the hose to the gauge can definately cause a delay and sluggishness in the reading at the dash.

When testing the vacuum, pinch off the line to the dash gauge and see if vacuum rises - if so, you have a leak @ the dash.