https://www.immihelp.com/cspa-calculator/
According to this CSPA calculator, the I-130 pending time (usuallly only 1 year) is removed, but the visa waiting time is counted in CSPA age. As you know the majority of time F4 are waiting for is not the USCIS approval, but the Visa, so a 15 year wait for visa would not be deducted from CSPA age.
