It is easy
check the accumulator
if : 0<= ACC =<51 then Acc =1
if : 51< ACC =<102 then Acc =2
if : 102< ACC =<153 then Acc =3
if : 153< ACC =<204 then Acc =4
if : 204< ACC =<255 then Acc =5

and Take a look to 8051 books for writing the code

Mehdi