The advance rate
Borrowing power derived from scenario loss.
A haircut on mark to market value is the wrong instrument for options. A long call and a short call may carry identical value and entirely different downside, so value alone contains no information about what is lendable.
Borrowing power follows from the same scenario loss that produces margin, with advance haircut η = 1.1. Positions whose scenario loss exceeds their value contribute nothing and can drive the expression negative, which is correct treatment: a short option book is a liability, not collateral.
An earlier revision applied the initial margin multiple of 1.3 to borrowing as well, which double counted the buffer and reported zero borrowing power against every long position. That looked like a property of options and was a property of the formula.