Flat rate expenses are not a tax credit , but are an allowance .
If you are claiming previous years , this allowance will be deducted from your gross pay before computation.
If you are claiming current year , this allowance will increase your Standard Rate Band .
So when you see a figure of say € 331 for Engineering Industry , this figure is an allowance and not a tax credit .
To simplify this , it can be converted to a tax credit of only € 66
Why not go ahead and claim , and set out your employment conditions on a separate note , and the tax office will decide your entitlement .