Well... I don't have access to a comouter right now so I could not understand fully the event sheet (only read the xml file) however it seems that when score is superior or equal to 25, you set the milestone to 50, then when equal or more than 50, you set to 75, etc..
I would have made it so:
Set milestone to (floor(Score/25)+1)*25
(Floor(25/25)+1)*25=(floor(1)+1)*25=50
(Floor(50/25)+1)*25=(floor(2)+1)*25=75
(Floor(26/25)+1)*25=(1+1)*25=50
Floor is a system expression that returns the number rounded down, and can be typed directly in the event sheet.
Floor(score/25) will return 0 for score between 0 to 24.99999.... 1 for score between 25 to 49.9999...
Adding 1 to that number then multipying by 25 should makes the milestone being the next 25 multiple you can reach.
If it is possible to actually score more than 25 in one tick(frame) then you should take that into account if the milestone gives you a bonus.