A glide is division.
Strip the drama away and a final glide is one division: height needed = distance ÷ glide ratio. Forty kilometres at 40:1 needs a thousand metres. Everything the computer does is a refinement of that line — but the refinements are where gliders end up in fields, because the ratio in the division is not the number in your glider's brochure.
The brochure ratio is measured in still air, at exactly best-glide speed, with a clean wing. Your final glide happens in moving air, at the speed your MacCready setting commands, with an afternoon of insects on the leading edge. The honest ratio — the one that decides whether you cross the fence — is the ground the glider actually covers per metre of altitude it actually loses.
Wind changes the ratio, not the glider.
The polar gives sink per second at each airspeed. Over the ground, though, distance is covered at groundspeed — airspeed minus headwind. The glide ratio over ground is groundspeed divided by sink, so a 20 km/h headwind takes a 42:1 glider down to about 33:1, and the same tailwind lifts it past 50:1.
Notice the asymmetry hiding in that arithmetic: a headwind hurts more than the same tailwind helps, because the wind takes a constant bite out of every second of a glide that headwind also makes longer. The defence is also in the polar: fly faster into a headwind. The classic rule falls straight out of the geometry — add roughly half the headwind to your best-glide speed: slide the tangent's origin upwind along the speed axis and the construction hands you the new best speed.
MacCready on final: choosing your arrival energy.
Between thermals, MC is a theory about the next climb. On final glide there is no next climb, and the setting quietly changes meaning: it becomes a dial for how steep — and how fast — you arrive. MC 0 is the flattest possible glide: maximum reach, minimum arrival energy, no reserve of speed if the last kilometres sink. A higher MC buys a faster, steeper glide that punches through sinking air and arrives with speed in hand.
This is why an MC 0 glide that "just works" is the most fragile object in soaring. Every metre of unexpected sink comes straight out of an arrival altitude that was already zero, and the only correction left — slowing below best glide — makes the ratio worse, not better. Experienced pilots fly the final glide at MC 2 or above and let altitude, not hope, absorb the surprises.
Compute the bet.
Start with the defaults, then do the two classic experiments. Add a 20 km/h headwind and watch how much altitude it costs — then raise the MC and watch the arrival drop further still: speed buys arrival energy and punch through sink, never reach. Then leave everything alone and add 15% of bugs: that is why the books say a marginal glide on a dirty wing is not marginal, it is short.
Margins are metres, not ratios.
A safety margin expressed as a ratio — "I plan with 38:1 instead of 42:1" — has a flaw: its value in metres shrinks exactly when you need it most, close to the ground. A margin expressed in metres does not care where the error happens. That is why every LX computer takes an arrival altitude: a number of metres, over the target, below which the glide does not count as made.
How many metres is a judgement, but the inputs to the judgement are concrete: the last kilometres are flown in the sink of other people's used thermals and in valley airflow; the wind estimate is least reliable exactly where terrain bends it; and an arrival needs enough height left over to fly a circuit rather than a straight-in surprise. Three hundred metres over flat land with an airfield is a common answer. Over mountains, with ridges between you and home, it is not enough — the question stops being glide ratio and becomes terrain clearance along the whole path.
The decision, in order.
Leaving the last thermal is the highest-stakes routine decision in cross-country soaring, and it rewards a fixed order of operations. First set the real wing loading and a bug factor the wing has earned. Then set the MC you will actually fly — not zero. Then read the arrival altitude over your margin, and believe the trend more than the number: an arrival that grows as you glide means the air is better than the model, one that shrinks means worse, and a shrinking arrival with the margin already spent is the signal to stop gliding and climb while there is still a climb to take.
And when the day is strong and the water is still in the wings, the whole calculation shifts in your favour — the heavy glider flies the same slope faster. How that works is the ballast article, one page back.