You should not use the worksheet on the back of the SSA-1099. It can be complicated to figure out even though it looks simple. Turbo Tax figures it all out for you.
When you enter 1/2 of your ss on line B it is not being subtracted from anything. It is being ADDED to ALL your other income to see if any of the ss will be taxable to you.
To see the Social Security Benefits Calculation Worksheet in Turbo Tax Online version you would have to save your return with all the worksheets to your computer. Or if you are using the Desktop CD/Download Software you can switch to Forms Mode (click Forms in the upper right) and click on SS in the list on the right side.
Up to 85% of Social Security becomes taxable when all your other income plus 1/2 your social security, reaches:
Married Filing Jointly: $32,000
Single or head of household: $25,000
Married Filing Separately: 0