A publication from the Social Security Administration explaining how it works -
2018 SSA Publications: How Work Affects Your Benefit
Read it and then if you have further questions, you can post them here and I will try to give you an answer.
i am assuming that you are younger than your Full Retirement Age -
As far as the math goes to keep you under this monthly amount - just divide $1420 by you hourly rate and this will give you the number of hours which you can work in a month and not go over the $1420 per month.
Example: if you make $25.00 per hour you would divide $1420 by the $ 25 per hour to equal 56.8 hours - at this regular rate of $25.00 per hour, it would take 56.8 hours in a month to reach $1420 in pay.
The above should work with regular hours and regular per hour pay, but if you get a bonus per hour amount for some hours in overtime, you would have to recompute.
It's Always Something . . . . Roseanna Roseannadanna