Market share variance

Fort Co. produces and sells three models of family cars. The basic model (the drastic), an upgraded model (the bomber), and a deluxe model (the cracker).All of the cars are priced to achieve a 6% markup on the standard cost.For the month of June, Fort Co. budgeted to sell 30000 units of the Drastic and so have 10% market share of the budgeted sales at a price of $10600 each. Fort Co. actually achieved a 15% share of the market, though the market has actually contracted by 5%.

How do I calculate the market share variance, please?

The formula I used is (actual sales-revised sales)*std contr.