Hello,
This is how I calculate reordering rules with multiple quantity

Possible Scenarios: 
Case-1: My On hand Quantity is 14, Minimum Quantity: 20, Maximum Quantity : 50, Multiple Quantity: 8
Maximum quantity should be ordered = Max qty – On hand qty = 50 – 14 = 36
8 *1 = 8 ( Now the max qty that should order is 36 and it does not meet maximum qty.)
8 *2 = 16 ( it does not meet maximum qty)
8 *3 = 24 ( it does not meet maximum qty)
8 *4 = 32 ( it does not meet maximum qty)
8 *5 = 40 ( it does meet maximum qty) ✓
Ans: 40 units will be ordered

Case-2: My On hand Quantity is 53, Minimum Quantity : 50, Maximum Quantity : 80, Multiple Quantity: 12, Sales order qty = 55
Maximum quantity should be ordered = Max qty + sales order qty – On hand qty = 80 + 55 – 53 = 82
12 *1 = 12 ( Now the max qty that should order is 82 and it does not meet maximum qty.)
12 *2 = 24 ( it does not meet maximum qty)
12 *3 = 36 ( it does not meet maximum qty)
12 *4 = 48 ( it does not meet maximum qty)
12 *5 = 60 ( it does not meet maximum qty)
12 *6 = 72 ( it does not meet maximum qty)
12 *7 = 84 ( it does meet maximum qty) ✓
Ans: 84 units will be ordered