Overriding Net to Gross Element

There needs to be an option to have 1 overall net to gross that overrides all elements.

Eg, an employee should get £363 net (£330 salary plus £33 tips), so we would like to select £363 OVERALL net, with £33 tips included and factored in.

We are currently having to manually work out the gross salary element then add in the £33 tips to achieve the net of £363.