Skew locks/rhinos are running at a pure arb condition. credit tails. I've never seen a "421" skew lock with a credit left tail.
It adds buying power when the tails are credits. I did a 20 lot SPX with a small debit left tail and it required $724 in initial under port-margin. Left tail was trivial; like $1800.
optionstrat is good and fairly conservative on term structure modeling. You will still need to alter the day count when trading tenors very close to expiration.
im' going to use it mainly for covered calls against positions opened by short puts if exercised so nothing real complicated. years ago i had optionvue but it's long gone.