If your outfit uses the Boeing OPT:
Go to your IAC for the runway you’ll return on if needing to come back in a hurry. Check the missed approach climb gradient for that approach.
Check different approaches (RNAV etc.) to find the lowest missed approach climb gradient, weather dependant for that runway. You can also check the reciprocal runway should the wind permit, if a more favourable missed approach climb gradient can be found. Add 0.6% if a turn is involved.
Punch all the info into the ‘Landing Dispatch’ section of the OPT. Then hit ‘Calculate’, then ‘Airport Info’ then ‘Landing Details’. Add 2450kg to the Go-Around Limited Weight and there is your RTOW.