Computing this can not be done.
not because of technical knowhow but lack of hard data.
Elon has changed the design of the upper stage just last week, AGAIN!!
Ok just a brief rundown of the changes in the past 6 months.
Booster went from 42 to 31 raptor style engines.
Of those engines the thrust has changed by being cut in half.
The second stage/ passenger compartment has changed as well. From 9 raptors engines to 4 raptor engines size and 2 smaller engines now to 4 Smaller Raptor engines and 3 small engines unknown thrust (I have a good idea based on goal requirements)
Why you may ask?
Well it all hinges on two drivers. The first is that all parts are to be used on mars and earth to make a multiuse rocket that has and high ratio of part interchangeability (90 percent plus). Second the main driver of launch costs are facility's and infrastructure at this point. By reducing rocket size and payload size he can continue using LC-39A and other like facility's with no large investment. Size change allows it to be used to launch the Orion pressure vessel with modifications.
Changes in the upper stage allow both mars and earth landing with a redundant engine/(s) in case of problems. (Just like the FAA requiring three engines for overwater flying until air body/engines can be certified by a safety record to allow less.
) Three engines also allow less granularity to be used while maintaining those thrust levels required to land on both planets.
So all I can do is a very crude analyses based off of other rocket data on the flight ranges. (subject to change at any moment)
So by holding a steady Gz load and keeping below Vne /Vno your looking at about 15 percent more fuel usage.
Maximum of minimum range of the upper stage would be 2000 +- 200 miles and using the booster with fly back capability would be 18000 +-1800 miles. This assumes a land with 40 tons of cargo/passengers.
He is also making a assumption that landing the second stage back on to the launching platform is due able to reduce maintenance and turnaround time. Something else I found out by using AREO braking you can get rid of a outstanding 10000 M/S of speed at 1.25 Gz something the space shuttle routinely did.
Disclaimer: This is all based on guesses because I do not have the real numbers and am not going to try until it is settled on what we have.