Preparation of flexible budgets

When prepare flexible budgets are using marginal costing method to compare actual activity level whether is correct ? and all the semi-variable costs must be resolved into their fixed and variable components whether it must using high low method ? example : actual units produced and sold 11,600 power cost - $17,000 - 10,000 units , $18,000 - 11,000 units , $19,000 - 12,000 unit my working to find power costs to compare actual sales & costs : $19000 - $17000 = $2000 , 12000 units - 10,000 units = 2000units VC per unit cost $1 FC = $19000 - ( 12000 units x $1) = $7000 11,600 x $1 + $7000 = $18,600 therefore $18,600 is total amount to compare with the actual sales & cost ? and so on others semi variable cost follow the same? thank you