Hmm, that's strange. Maybe I'm misreading it, but I just
read Fidelity's margin requirements over again and it does seem like your margin requirement should be 60% if your TSLA holding is 98% of your holdings.
"Example: If you purchase $20,000 of marginable stock with a 30% house margin requirement, you would need to initially deposit $10,000, which is the 50% Fed requirement. You would not need to deposit additional money beyond the $10,000 because the house maintenance requirement is below the 50% Fed requirement.Let’s say, however, the security purchased now makes up 80% of the gross market value of your portfolio. This security would be subject to an RBR add-on of 30%, bringing the house requirement to 60%. Since the account has a maintenance requirement higher than the Fed requirement, you would need to deposit funds to meet the higher requirement, rather than 30%. In this example, the security purchased increased the house maintenance requirement to 60%, requiring a deposit totaling $12,000. This amount is equal to 60% of the purchase price."