Could someone confirm my understanding of how a margin account works with interactive brokers?

Lets assume I'm trading US stocks which require 50% margin.

Step 1: No Margin used

Equity: 100,000
Debt: 0

Exposure: 100,000
Margin Requirement: 50,000

Step 2: Margin used

Equity: 100,000
Debt: 75,000

Exposure: 175,000
Margin Requirement: 87,500

Step 3: Portfolio Falls 30k, triggering a margin call

Equity: 70,000
Debt: 75,000

Exposure: 145,000
Margin Requirement: 72,500


I guess what I'm wanting to confirm is my understanding of the margin requirement as the portfolio moves around. In step 2, even though the margin requirement of 87,500 is only 12,500 less than the equity level, the account actually needs to lose > 25,000 in order to trigger a margin call.

Thanks all