Marginal costing is variable costing only. No fixed cost is considered for decision making. It's cost of extra production, best mix, key factors and capacity availability formula.
1) sales_variablecost = contribution_fixedcost = profit
(2) profit volume ratio = contribution / sales*100
(3) breakeven sales = fixed cost / pv ratio
(4) margin of safety = acutal sales _ break even sales sales
(5) desired sales = desired profit /pv ratio