Formula for Activity

Hi team

I have a confusion in understanding, how activity formula will work.

Currenly for my client, there are some a parameters which I have decided as activity such as steam (ton) , water (M3), power(Kw), furnace oil consumption(ton) and labours(hr).

Now the question is how to I give them provision during confirmation,

1. Should I define Duration as one parameter and assign formula to it by linking it with respective parameters.

2. Should I give client opportunity to enter labour, power, etc during confirmation but in this case no formula required.

Please help me out, I am totally confused over here in terms of understanding it