I retired from T about a year ago and took the lump sum pension which is doing quite well right now. That could change because a portion of it is invested in the stock market. To answer your question about how much you will receive you need to understand Net Present Value and how lump sum is calculated.
NPV has:
R - Net cash outflows during a single period (what the company would pay you each month/year)
i - Discount rate or return that could be earned in alternative investments (IRS segment rates posted in November of previous year)
t - Number of timer periods (how long you are expected to live based on actuary tables)
The net cash outflow to you is the payments you would get if you took an annuity. That is based on how much you have accrued based on company contributions. I hear that is changing, but it should only impact future payments made by the company.
The discount rate is three discount rates (short, medium and long term), so one lump sum is determined for short term, one for medium term and one for long term (if you life expectancy (t or term) allows for all three calculations) using the R (cash flows). These three numbers are added to determine your lump sum.
If you retire today and decide to take the lump sum tomorrow, you will use the interest rates from last November (which are very good because they are low) to calculate your lump sum with your current age used to determine the term.
If you retire today and decide to take the lump sum at the end of the year (waiting to see if interest rates are higher or lower), you will use the interest rates from last November or this November based on your choice to calculate your lump sum with your age at the end of the year used to determine the term. This means that if you decide to use last November's rate the lump sum will be lower because you have grown 10 months older and your term was reduced.
If you retire today and decide to take the lump sum 10 years from now (which I don't recommend), you will use the interest rates from November 2030 to calculate your lump sum with your age 10 year from now to determine the term. Even if interest rates are lower in 10 years (which is unlikely), you have erosion on the lump sum because the term would be reduced by 10 years.
I hope this makes sense.