how to set a variable x to apply to the same formula across a spreadsheet

Cumbersome title, sorry.......and sorry if that this is a simple one.

I use a spreadsheet to collate the hours worked on different clients and then apply the same formula to each client to work out deductions for corporation tax etc. Simply £(amount)*30%. Occasionally I need to change 30%. I want to call the % figure x and then simply change the value of x once so it applies to all of the clients. Can't quite get my head around Formulas>Define Name>Scope if that is the right way to do it.

Yes, you can do that like that -- in this case the name is referring to a constant, so

Formula:

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then in your cells:

Formula:

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you could also store the 0.3 in a summary tab in your file, and reference that explicitly -- e.g.

Formula:

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where Summary!A1 holds 0.3... and if you prefer to persist with "x" you can name Summary!$A$1 "x" (i.e. refers to a cell, rather than a constant)

Thanks so much for the prompt reply