One minute of LATITUDE is approximately a nautical mile. Use your
fingers as a crude caliper to measure distances on the chart.

Double the distance and multiply by 100 for 30:1
Triple the distance for 20:1

I'd set my GPS for STATUTE miles then use the above math while flying
the club's ASK-21 for a conservative glide for home.

Use an E6-B to calculate L/D. Some high end aviator watches have one.

Distance in NM vs altitude and the pointer indicates glide ration
needed to make the goal. Update every few miles and if the L/D
remains constant, then you're on glide. If it gets smaller, then you
have it in the bag.

I recall the huge amounts of heads down time in the pre-GPS days while
on a tight final glide or over unfamiliar terrain as I tried to figure
out precisely where I was.

-Tom