How to decide how much rent you charge?

[Victor Martinez Lazaro]

Other than looking at similar looking properties how can one exactly decide how much they should rent?

Further on, if I were to have property that I have done some upgrades on, or overall improve, how do I figure how much more rent I should charge? 

Using some basic knowledge I know the more expensive the project the more I should charge, but then how do I find that sweet spot where the project pays for itself and other things while not turning away potential tenants?

Although these are pretty similar questions any and all replies are greatly appreciated!