The layout of the box is shown below:
HDG
TRK
FL
IAS
SALT
The above was used for medium/high level nav where the FL and hence IAS was leg specific. For low-level the IAS for each leg would usually be calculated to give a fixed standard ground speed (7nm/min, 4nm/min, 3.5nm/min, 2nm/min)
The photo in the link at the start of the thread depicts a low-level map/route with a leg distance in the second box of 42nm. The leg time below it showing 21:00 ie 21 minutes (at 2nm/min). No IAS or alt/FL data is needed at low-level. An on-the-day wind-corrected heading was usually put above the track in the arrowhead of the symbol.