Count down please HELP

[Jaharul]

Hi All

I want to subtract a rolling form which is updated daily against a budgeted figure but i jus cant get it to work please help. Its basically like a stock system when an item is sold the quantity in stock is reduced.

Many thanks to the helper.

 

[Jaharul]

ive checked n i cant see were the problem is coming from but thank you for all your help uv been brilliant