How to calculate HELOC and cash out refinance?

[Ethan Kramer]

Terribly sorry if this has been asked before, how do I calculate the maximum cash-out refinance amount I can do if I already have a HELOC on my primary residence?

Primary residence is valued at $375k

$150k mortgage balance

Let's say I want to open a $150k HELOC on the primary residence to start BRRRR cycles

What is the maximum cash-out refinance amount I could do for the primary residence? 

Is it 1) Primary residence - mortgage - HELOC = cash-out refinance, or 2) Primary residence - mortgage = cash-out refinance?