Adding spare bays to best ranked products

[Peter Davison]

I have a list of bays in a column (a) that add up to 18 across 4 rows (see below)
I have another column B that has used sales percentages to allocate bays and also used rules on rounding and these add up to 16
I want to add the difference 2 bays (18-16) to the best ranked products in column c (1 and 2) in a separate column. Based on a switch rule of a cell that says use 0.5 or 1.0 size bays.
Is this possible in a formula?

a - b - Rank (Column c)
4.0 - 4.0 - 2
3.5 - 3.0 - 3
4.5 - 4.0 - 4
6.0 - 5.0 - 1

18 - 16

Thanks for any help.