making a spreadsheet for student coursework making

Assuming late % to be calculated in G12 onwards

G12: =IF($A12="","",LOOKUP(MAX(0,F12-$G$1),{0,7,14,21,28},{0,0.1,0.2,0.3,1}))
copied down

formatted as %

not entirely sure I followed the date requirements - you may want {0,1,8,15,22} rather than {0,7,14,21,28}