Hello! Zushuka has a spawn cooldown of a minimum of 19 days and a maximum of 29 days.
In case nobody kills her and her spawn is skipped, we must use a math on her cooldown to determine the next spawn: we must deduce that she appeared and nobody killed her on the first or last day, that is, on the 19th day or on the 29th day.
Let's assume that a Zushuka was killed on March 1st. The next one was supposed to appear between March 20th and March 30th (between 19 and 29 days), but nobody killed her. The date of the next Zushuka would be between April 8th (19 days after March 20th) and April 28th (29 days after March 30th). Then you must look for Zushuka between April 8th and April 28th (between 38 days and 58 days from the last officially known death).