One possible approach:

B3:
=IF(COLUMNS($B3:B3)>5,0,MIN(8,B$6))
copied to H3

B4:
=IF(COLUMNS($B4:B4)>6,0,MAX(0,MIN(2,B$6-8*(COLUMNS($B4:B4)<5))))
copied to H4

B5:
=B$6-SUM(B$3:B$4)
copied to H5