My take:

500 per month or 6000 per year income and 5% return is equilivent to starting with 76643 at the beginning of year 1, taking out 6000 per year, and the remaining capital earning 5% per year. At the end of 20 years you will have taken out 120000 and be left with zero capital.calculation.

simple excel spreadsheet

year start ...end ...out ..income
01 76643 74325 6000 3682
02 74325 71891 6000 3566
03 71891 69336 6000 3445
04 69336 66652 6000 3317
05 66652 63835 6000 3183
06 63835 60877 6000 3042
07 60877 57770 6000 2894
08 57770 54509 6000 2739
09 54509 51084 6000 2575
10 51084 47489 6000 2404
11 47489 43713 6000 2224
12 43713 39749 6000 2036
13 39749 35586 6000 1837
14 35586 31216 6000 1629
15 31216 26626 6000 1411
16 26626 21808 6000 1181
17 21808 16748 6000 940
18 16748 11435 6000 687
19 11435 5857. 6000 422
20 5857. 0.0000 6000 143

total withdrawal 120000

start = beginning of year balance
end = end of year balance
out = amount withdrawn per year
income = earnings at 5% of the average balance during year