Calculating a range based on a fixed percent

I need to calculate a range (tolerance limit) for a set standard based on a set percentage. For example, if I have a standard of 30 with a tolerance limit of 4%, I need to create a cell that will calculate and display the range of 28.8 - 31.2. Both the percentage and the standard will change based on what is being tested, so I need a calculation that will automatically display the tolerance limit based on whatever
standard and percentage appears on that line in the spreadsheet.

Thanks.