I have used vroute (premium version) for fuel calculations. It's quite handy even if you are not flying in Vatsim. It's Navigraph compatible, provides appropriate route planning tool, weather information and atc coverage information if flying in vatsim.

Regarding fuel planning, the programme has CS B753 as available aircraft type but not CS 752. However they have PSS C752 and I think that is close enough.

Regarding the fuel burn, here is something that can be used as thumb rule assuming long range flight, full payload and cost index 50 (accumulative figures):

First Hour                4.120 kg
Second hour            7.680 kgs
Third hour               11.160 kgs
Fourth hour             14.880 kgs
Fifth hour                18.750 kgs
Sixth hour               22.860 kgs
Seventh hour          26.880 kgs
Eighth hour             30.960 kgs

Even rougher thumb rule would say that 757 burns approximately 4.000 kg / hour. And according to my experience captain sim 757 seems to burn fuel approximately on the above mentioned rates.

Nikke