Long Term Capital Gains

[William Behm]

If I am married filing jointly and we make $250,000 income during 2018 and then sell an asset for $750,000 Long Term Capital Gains, what will be the tax rate on the capital gains?

Trying to understand:

1) Are Long Term Capital Gains taxed at a progressive rate (part of the gain at taxed at 15% and the remaining that exceed total income of $479,000 taxed at 20% or does the entire amount get taxed at 20% if you cross the $479,000 income threshold)?