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