For Boeing a/c look in the early pages of the QRH. There is a table of Optimum Alt v. gross weight.
If your looking to file a level for ATC; take the ATOW, subtract the Climb fuel, which in light/medium cases, is 1500kgs and this will give you TOC weight and thus the initial filed ATC FL. You can then also assess when the step climb will be by deducting the fuel burn/time for a 2000' or 4000' step climb.