This questions pertains specifically to Social Security. If I get paid 1000 a month, take out 200 (20%) untaxed to 401k with 800 left over, how much should my Social Security tax be? Do I multiply 800 x .062 (rate of social security right now)?

Social security and Medicare taxes apply to your base pay before the deduction for the 401(k), so in this case your SS tax is $1000 x 6.2% = $62.00, and Medicare = $1000 x 1.45% = $14.50.

In case you're wondering why the 401(k) contribution is taxed for SS amd Medicare - it's because when you make withdrawals later on in retirement the proceeds are not taxed for SS and Medicare. So you pay now instead of later. Income tax is different - you avoid paying now on your 401(k) contribution but then you have to pay later when you make withdrawals.