not too sure I think it depends on the firms criterior ,but say if you stayed with capital spreads at £80.00 a point and the market gaps 65 points then you would be liable unless its a secured/guarenteed stop typically 3x fat spread min stop distance at some places then your leverage can be 100's to 1
so if you are overleveraged and the market gaps through then you will be liable , hence 50-1 is sensible, 400-1 may seem good bang for your buck but...
with Oanda because I use tightish stops the price has zipped straight through say 15 beyond a couple of times and i was expecting heavy slippage, but ive always been filled on my level, surprised cos I couldn't of traded the move , so I dont think you will get any margin call at 50-1 , 20 points, 400-1 though with a sloppy broker. .... dont know.
like Oanda say more than 50% of clients are still there 5 years down the road with them, so that says something. does sensible margin help, yes, and good spreads, reliable orders, yes. probably has a lot to do with it.
thinking about your 400.1 you would be chased for any shortfall for sure.
400-1 5k, £160-200 per point lol. im sure they would love people to use that 30 points and your £5 grand is gone... gaps 60 points your 5 k is gone and you owe them 5 grand on top.
Very scary. like the film " Gone in 60 seconds. "