In CREOTIVE we mostly use the agile methods and approaches in order to get the estimation more correctly.
Firstly, the project manager invite the developers/designers to the brainstorming corner and then read to them the requirements one by one. Each team member have a pair of Planning poker cards and for every requirement everyone drop the card in which is shown the estimate hour. Basically, poker planning uses Fibonacci sequence because the information that we obtain out of estimation grows much slower than the precision of estimation.
In every step team members discuss the hours if they are different from each other. For example if for a certain requirement, Developer A has dropped 8 hours card and Developer B has dropped 2 hours card then they will discuss why it takes so much time, Developer B will remind the team that a kind of requirements is done in the past and we can reuse it etc..
In the end of it, project manager collect the estimation for each task/requirements and provide to the client, team and anybody else involved in the project.