The maximum fuel is your MTOW minus ZFW. Then make sure that you are not going to exceed your MLW on arrival (you are limited by one or the other) and that the LDR is not greater than the LDA.
There are some other considerations - trip fuel is now going to be increased (the 737 burns 2.5% of the weight of the tankered fuel in additional trip fuel every hour). Also, if you are doing a turnaround at the other end, cold-soaked fuel frost can be an issue.