I would never, ever design a data base where the employee types in such data. It's too vulnerable to data entry errors or even fraud.
Instead, set up a table of employees with such data entered as fields in the employee table.
When an employee, creates a new invoice, use their account name to look up this data from the employee data to use in your calculations.
The contractors are paid different rates depending on the job that they do. How they complete invoices now is copy every thing in to Excel and enter their specific multipliers then print the invoice from Excel. I would like to create an invoice that basically is blank with just their names on it so that when they chose the jobs for them to be paid pick their invoice layout at they can type in the %rate that they are paid for that invoice then have a total at the bottom.
This percentage changes.
The method that I have already described can be used for this purpose. If the rate is specific to the type of job rather than the specific employee, the only difference is that you are looking up this rate from a table of job types.
If each employee is paid at a different rate that is determined both by the employee and the job type, this can also be done via a table that stores an employee ID, a job type and a rate. You link it to invoice by both employeeID and Job type and when both employeeID and JobType fields in an invoice record are filled in, the correct rate is looked up from this table.