If the spouse has no earnings then there is nothing to add onto. The base is the wage earner's FRA (aka PIA) benefit divided by two. If the spouse, Arwenmark, claims early, then the early claim penalty is applied to the 1/2 she would have drawn if she waited until FRA.
Here's an example: I have an earnings record. On my record alone, it looks like my benefit would be less than 1/2 of my husband's FRA benefit. It doesn't matter if he retires before he reaches his FRA. I am still entitled to 1) the benefit on my record and 2) enough of my husband's benefit to bring mine up to 1/2 of his.
But wait, what if I did not have an earnings record? Then I would be entitled to 1/2 of his benefit. Note that I end up with the same amount regardless of whether there was an "add on" as in my first example, or no "add on" as in the second example.
Arwenmark is the second example.
Next step: She could have 1) claimed the spousal share (1/2) at HER FRA. or 2) claimed the spousal share before she reached the FRA. Arwenmark chose # 2. Thus, she will receive an amount equal to 1/2 of Husband's FRA benefit MINUS the penalty for claiming several years early.
In this case, I'm not sure if Arwenmark is saying her H is entitled to $2500 NOW or whether he would be entitled to $2500 if he had waited to claim at his FRA. The figure we need to work from is the amount her husband would have received at HIS retirement age, the "PIA" (even though he claimed early). If we had Husband's PIA and knew exactly the number of months Arwenmark claimed before HER FRA, we could calculate the correct amount down to the penny.
https://www.ssa.gov/OACT/quickcalc/s...tml#calculator