You do not need to do anything with that worksheet on the back of your SSA1099. Just enter your SSA1099 information into the software as it appears on the document and allow the software to do its job. The software will do all of the calculations.
Go to Federal> Wages & Income>>Retirement Plans and Social Security (SSA1099 and 1099RRB) to enter your SSA1099.
Social Security will show up on lines 6a and 6b of your Form 1040. The amount of your SS that is taxable will be on line 6b.
You only quoted part of what line 3 of the Social Security Benefits Worksheet says. Line 3 says "Add the amounts on Form 1040, lines 1z (before adoption benefits exclusion), 2a, 2b, 3b, 4b, 5b, 7, and 8." It is the total of all those lines from Form 1040, not just line 1z.
I don't know what you mean by "1040 Line 6 worksheet." There is no such worksheet. Do you mean the Social Security Benefits Worksheet? If you still think the calculation in TurboTax is incorrect, where exactly do you see 51,188 -- on what form or worksheet and what line number on the form or worksheet?
There is no line 6 on Form 1040 or Form 1040-SR. There are lines 6a, 6b, and 6c. If you are questioning a calculation, you have to be precise about what figures you are looking at.