Grading Formula

Each item gives you a value of 0, 1, or 2 at the end of the game, and spills give you a value of either 0 or 2

Not Hidden - 0 Points
Hidden Incorrectly - 1 Point
Hidden Correctly - 2 Points

 Uncleaned Spill - 0 Points
Cleaned Spill - 2 Points

This means there are 68 possible points, and percentages round to the closest number, so the amount of points required for each grade is as follows

A+ - 68
A - 61
B - 55
C - 48
D - 41
F - 0

Since incorrectly hidden objects give you a single point, odd numbers can be filled out by cramming an item into the closest available location, which means these are the required number of items to receive each grade. Keep in mind it may be more efficient in some cases to incorrectly place 2 or more items if it saves time over placing an item correctly.

A+ - 34
A - 30 & 1 Incorrect Item
B - 27 & 1 Incorrect Item
C - 24
D - 20 & 1 Incorrect Item