Let's parse this completely-unattributed blurb, shall we?
150-250k dead in the two years following emancipation (later simplified to 200k for the final tally)
15% of 480k died in transit, rounded to 75k.
30% of 480k in slave raids, rounded to 150k.
Yes, indeed that does add to 425k. Congrats on finding an unnamed source that presents a number that conforms to what you're trying to find.
It's just too bad that it ignores the millions of premature deaths of enslaved people with a patently-false hand-wave of "Death rates once in America tracked those of the rest of Americans". I've already shown that this is completely wrong. Life expectancy for enslaved people was half of what it was for whites. And frankly, life expectancy shouldn't really matter anyway. Death is the denial of a free and self-directed life. Half a million people were denied that life the minute they were taken against their will to the US, and millions more were denied that life the instant that they were born into slavery.
What kind of source counts the number of deaths due to the delivery of slaves to the US, as well as the deaths directly after the abolition of slavery, but ignores the hundreds of years in between? I'm sure you were excited to find some suspect math that confirms what you were hoping to find, but please put a little more critical thought into these things.