Lets say your budget is in cell A1 and is 1000; the amount spent is in cell B1 and is 750. The formula for the percentage spent is =B1/A1 = 750/1000 formatted as a percentage = 75%

I'm not sure which value you want to be absolute ... if it's the budget itself, then that would be $A$1 and the formula becomes =B1/$A$1


Regards, TMS