Hi Belleke,
I drafted a solution that appears to be quite similar to Bosco's (well I mutliplied by 1000 rather than 0.5, but otherwise it is very close), except that I made a table in your "Reservaties" sheet, because it makes the formula more readable, and it is more flexible, as the table automatically expands when you add rows (or columns), while you have to change your formula each time your data size changes.
Catherine