Modern margin lending isn’t based on fixed percentages. It is dynamically calculated based on the actual underlying securities. So (to make up numbers) maybe TSLA is only eligible for 8% margin but BRK is eligible for 62% margin. You can google “portfolio margin vs reg t margin” to learn more.
So, the actual number will depend entirely on what’s in your portfolio. It also depends on whether you want to use it to buy securities or withdraw cash.
Looking at my account my “cash available” — that is the amount of $$$ I could transfer to a bank account — is around 0.8x my portfolio (e.g. 80%). My “buying power” — the amount I could use to buy securities — is around 4.7x my portfolio.