First answer is always: don’t try this at home 
Nevertheless, it will not harm you to “study” how the mechanism work without putting real money on the table…
Start with the IB guide and be sure you fully understand it. Run some simulation with a spreadsheet in order to “play” with real numbers…
Start analyzing RegT margin, whose requirements are fix and easier to understand (initial margin 50%, maintenance margin 25%)
Both your examples should work as the ratio between ‘equity’ and ‘equity + loan’ in order to buy should be >= 50% (where ‘equity’ is the total value of shares held and ‘equity + loan’ what “your belongings” are worth…)
Below a possible evolution of the situation until the triggering of a margin call
