In the POH you will find how much fuel it takes to climd at a certain weight to a certain height at so and so climb speed. Use this number to Top Of Climb. Then derive whatever leaned fuel consumption - at your chosen cruise altitude - for the duration, once again from the POH. Or if you would like to build in an extra safety margin, use the rich mixture fuel burn even though you're planning on flying leaned. Add the reserves on top of this.