LF with make or buy

I would like to ask about limiting factor with make or buy decision question.
First is to compare variable cost of making per unit and varible cost of buying per unit. Then, we will get the cost saving per unit and divided by the limiting factor usage per unit, followed by do the ranking (priority for internal manufacture) and comes out the optimal production mix. The remaining balances will be bought from sub-contractor.

How about when we calculate the profit,

We need to use cost savings per unit X production mix volume= total cost saving and less the fixed cost in order to calculate the profit

OR

we need to use:
own production: (selling price -variable cost of making) X production volume made by own =contribution

Add:

buy in units: (selling price- variable cost of buying) X production volume made by sub-contractor
= contribution

Then, become total contribution and we less fixed cost= Profit?

I also gt one doubt about whether selling price – variable cost of buying =contribution or not?

Thanks