Yes, IT DEPENDS on far too many variables to come up with one simple formula. If, for example, the flight were to be conducted at one Flight Level throughout, some fairly simple formulae might emerge which were reasonably valid over a modest weight range. If, on the other hand, increased weight delayed a step climb, or prevented it altogether for a longer range flight, a whole new set of parameters emerge.
The only realistic method is to run a series of flight plans at different weights, and establish the relationships. The better computerised flight planning programmes provide data for the expected weight, and plus/minus Time and Fuel adjustments for ZFW change. Similarly, adjustments are provided for 'Off Optimum' Flight Levels.