Hello Shamima Sultana and thank you for your response to my query.
The formulas in my spreadsheet that are giving me negative and/or a visible zero (0), ie not a blank cell, results are:
=IF($G$10>200,0,CEILING.MATH($F$10-$G$10,50)/50) and
=IF($G$12>6,0,($F$12-$G$12))
in which Column F...