For the purpose of entering in a number for 401k monthly slot you can use one of two methods for making an estimation.
This is based off the philosophy of how 401k were designed. It assumes you are going to max out your contributions all the way to 65.
Take the average of their 5 highest years of salary.
Take that number and multiply by .60
Enter that number into the calculator
Or you can use this 401k calculator and enter the parameters to get a total. You will then need to make a simple calculation to get the monthly number once you get the total from the calculator.
Once you have the total you will need to break that down into a monthly number.
Total / 20 years = annual total
annual total / 12 = monthly total
enter the monthly total into the calculator