A while back there used to be a site called MMOmorsel. One of the many great features they had was a Legendary Item simulator. You could put in all your legacies you wanted and build it up to see how the stats would be, and how many points you needed to spend.
The site has stopped updating and that tool is no longer useful anymore. Well I have found a updated one! It is a plugin that was recently updated from Lotro Interface. You can bring this up in game, and build a “virtual” legendary item and play around with the stats on it.
This even covers bridles.
Later on they plan to add relic support as well.