You have a few issues here, the fuel estimate doesn’t factor in aircraft weight.
If you were heavy then your fuel won’t get you as far.
as said above M0.88 is too fast, assuming you had a high weight you should have been at about M0.80 for a while, gradually increasing speed to M0.85.
either height you mention above is a fine cruise altitude for the 777, however step climbing is essential so as your weird drops, your speed and height increase. If you are unsure on how to do it properly, visit #tutorials where you will find the answer to your questions.
You should also check out this tutorial on using trim.
Okay! Thanks for the answers to all of you.
But I have one more question. When I fly slower with my aircraft. And I am on criuselvl, then the nose of my airframe goes up and the tail goes down (so that the aircraft does not sink because of the lower speed)
Aircraft fly nose up anyway, particularly when heavy. Anywhere between 2-5° up is healthy. That’s why step climbing is so important, if you’re near MTOW and you climb right up to 360 then you’re planning to fall out the sky. Try stopping the climb around FL300-330 and then increase your height as the weight falls off.