You martingale for a short period of time and few bets try more and Lets see if you'll get the same result as positive profit. But if you do that you must have a big amount of balance to cover your continous losses during martingale. And don't forget martingales work not only for you they works for website as well.
Well there are always other martingale based strategies, just need to calculate some of them

Indeed, calculation is aslo good idea but you cannot often calculate and sometimes your calculations aren't go right cause of house edge