Change Formula In Column When Cumulative Total Reaches Certain Value?

i am trying to create a spreadsheet that will automatically plan a days work in manufacturing based on the items the factory is making, the different items obviously have different hourly production targets and id lke to create a spread sheet that knows how many we want to create what the targets are and how long it will take. i also want it to be able to know when it has reached the needed total and automatically plot the targets for the next product, so what i need is a way to change part of the formula when the column reaches a set culmative total