Count calendar dates
A Seoul–Gyeongju transfer and a same-day continuation to Busan can use two trains on one date. That is one selected pass day, although seat-reservation limits still matter. A separate Busan day trip or return to Seoul on another date can create the second or third useful day. Draw the itinerary by date before selecting a product.
Price the marginal third day
Compare the official increase from the two-day to three-day product with the live tickets that would fall only on the third date. If those tickets cost less than the upgrade, keep the two-day pass and buy the third date separately. The all-pass assumption is convenient but not always the cheapest complete setup.
Do not compress the holiday to save the pass
A mathematically cheaper pass can become poor value if it forces two distant journeys into the same sightseeing day or exceeds the practical reservation pattern. Preserve time at each destination first, then use the calculator to price the rail days the itinerary can actually support.