I’m sure most people have experienced the problem of starting a piece of work only to discover that a slightly different composition would have been better.
The way I get over this is by painting a watercolour sketch of the composition in the middle of a sheet. I can then play around with the boundaries until I achieve what I want, this can then be used for the final painting knowing the composition is just what I want.
For commissions it can also be a useful way to comunicate the potential finished paintings to the client.