Mortgage Calculator Formula

Hi there,
I’m hoping someone can help me. I appreciate this might sound simple for other people however formulas aren’t my strong point.
I’m trying to create a calculator so that people select whether it is a joint or single application, move the slider to their annual household income and then the calculator will output an estimate of how much the customer could potentially lend.
I have created the boxes to opt for single or joint and the slider to select salary. However I now need to input a formula for the outcome and I’m totally stuck! Single application is 4.5 times salary and joint 3.5 times salary.
If anyone could help I would be so grateful.
I have tried to explain that best I can hope it makes sense.

Thanks in advance.