Formulas to durations or equivalent

I am building a production scheduler with a built in calculator where by the operator can adjust the labour allocation to test the duration output. 

My problem is Smartsheet won't allow durations to be edited by way of a formula.

Also I am wanting the variables to be as follows:

Completion Date - Constant (fixed) Variable

Duration - Dependant Variable (dependant on formula input)

Start Date - Output (calculated backwards, completion date less the duration)

I can quite easily achieve this by disabling dependencies however I need dependencies to include and programme sub-deliverable and subsequent tasks.

If you wish to contact please send to huxley@apollofabrication.com.au not to accounts.

Thanks,

Huxley