Looking at this request this isn't as simple as doing one calculation because of the various pipeline stages unless you can use operations hub to do this live calculation.
But if you remain on the current package you are on you will need to create a weighted forecast property for each stage in your pipeline as a "holding" property. So Stage A: Amount * weighting, Stage B: Amount * weighting. then you would create a workflow that will trigger when a person moves into each stage, to replace the custom amount field (main property for reporting) with the weighted "holding" property for that stage.
I hope that makes sense but there isn't a once off way to do this.
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