herm and gold,
Exchange minimum (for outright and spread) margins are determine by comparing historical volatility, price, and interest rates to current volatility, price, and interest rates. (weighted in same order). Very generally, for outrights you can expect a margin change every 20% (approx) price change. Margins ‘trend’ in that they tend to continue to rise or fall instead of bouncing up and down. Also, they tend to rise faster than they fall and when they do get to extremes they tend to come off the highs more readily than they rebound from extreme lows – ie sometimes extremely low margins can persist for longer than the ‘data’ would indicate.
The margins offered by individual houses, ib’s, etc. are a whole different story. A percentage offer / use exchange minimums, but most quite arbitrarily created their own margin lists.
Note that all of the above is generalization – ie rules of thumb.
In the long run, you’ll be much better off creating your own levels of leverage instead of ‘maximizing’ margin – ie have more than adequate capitalization for all positions you’re going to enter and only load up to allowable leverage in REALLY special situations.
All the best,
zdo