In that case you will need to apply for a child arrangements order on form C100. You will have to attend a MIAM first - mediation information and assessment meeting - and if your ex doesn't engage with mediation, or doesn't respond, then the mediator will sign your form C100 and you can proceed to apply to court.
The cost to file a C100 is £215 - the cost of the mediator will vary according to the area you live in.
Once an application is made to the court, the court will invite CAFCASS to carry out initial checks and speak to both you and your ex to find out what your views are, they will also do background checks (police and social services) and then write a safeguarding letter to the court to make recommendations as to what should happen next. The court will then set a hearing - usually within 6 weeks of getting your application.
If the court decides it is in the children's best interests to spend 50/50 with both parents, that is what they will order - but the reality is that the current split is very close to that anyway, and the court is reluctant to change things unless it is in the children's best interests to do so.
Given your current split, you probably shouldn't be paying as much child maintenance as you currently are. The online calculator is, of course, a mere legal minimum that the law expects you to pay, but where you are having the children for such a large proportion of time, the amount should be reduced accordingly. If you had the children on an exact 50/50 split, neither one of you would pay the other child maintenance, you would both be responsible for providing for the children whilst they are in your care.
Does that answer your query?