Can someone check my math here?

Current 'rate' of fiat entering bitcoin  is ~ $650*1800 = $1.17 mill.
Before halving that was ~ $650*3600 = $2.34 mill.

Before that it was ~$450*3600 = $1.62 mill.

These numbers are necessarily approximate, since price fluctuates rapidly day to day, so judicious choice of 'mean' price is essential for even approximate correctness.

Hypothetically if bitcoin were to have a few banner years until the next halving we might expect a total of $10 billion to escape fiat into bitcoin over the next 4 years ...

total coins issued until next halving = 2,625,000 btc
$10 billion / 2.625 mill. = $3,809

(N.B. that's simply asking what $10 bill of new money would have to pay to buy all the new coins off-the-shelf (out of the mines) until next halving and discounting any mania or hoarding effects such a run would have on miner or current hodlers psychology)

At current typical rates (lower limit) say $2.5 billion wants to move into bitcoin in the next 4 years ...

$2.5 billion / 2.625 mill. = $952