Dissecting the many Transaction cost's

After blowing 2 accounts I am rightfully obsessed with risk management especially after realizing how not accounting and even understand all the many transaction cost that play a role in slowly assassinating one’s account.

The topic I would like to dissect at the moment is understanding Transaction costs related to Leverage.

Question 1:
Which leverage is used to calculate my leverage-related transaction costs?
a)Leverage I opted to use when selecting the amount of leverage I would like to use?
b)True Leverage which is the notional value(the number of actual units traded) divided by the trading accounts balance?

Question 2:
Cost as % of Margin Required is taken as a percentage from my account balance or from the Margin Required?

Thank you in advance.