If its and LX then its and EASy, go to the alt page and you will see the optimum and maximum. Cross ref with the QRH figures for weight and altitude.
I found on the Falcon that M0.8 is pretty good but you can cheat, climb at profile, then as soon as you can make an optimum height at LRC go for it, even slowing to M0.02 < than LRC book figures will get better than book if time is not the issue.
If you really need to milk it the obviously picking the best winds work, I also found "block levels" and drifting climbs at < 100 fpm climbs can work really well.
It is possible to depart for destination with "Check reserve fuel" showing (obviously having an enroute alternate) and land at destination having made 1500lb more than reserve.
4200nm is entirely possible in an LX but you have to know the destination has close alternates.