You can't calculate it because it depends on the daily movements.That's why I am trying to learn how to calculate SPXU values at various stages of S&P.
Start with 100, one day is +25% and the next -20%. Index is still 100, but SPXU went +75% and -60% which brings it to 70. If the 2 days are flat, then SPXU would be worth 100.