The following spreadsheet will supply the proper corner weights to yield equal rear tire loading, on launch, with a non-IRS car at the instant when the front wheels lift.

Obviously, you will need wheel scales to use the output and to insert the proper values into the spreadsheet for your car.

The percentage front roll stiffness value I've inserted is reasonable, though you might want to change it if you feel it does not accurately describe your car. (Use the spreadsheet on Page 28 for measurement of percentage front roll stiffness.)