Yes indeed, I know about Topcat and it will probably be my final choice after all (in spite of that silly name!). Because The calculation to reach a proper trim is in fact rather complex.

But for now, I'm just trying this CS model out, so I just need some raw, but somewhat, vice suggestions based on experience!  

Last time I flew BA B767 RR. A 2 hour flight, from Zürich to Rome with 110 passengers and a GRW of 255.4. I chose C/G 24% and grabbed the calculated CDU Trim suggestion of 2.85, which all resulted in Takeoff-abortion because of loud and insistent Stabilizer alarm!
The 24% was just something that I picked up from another thread - but obviously this most be a wrong value or at least based on some entirely different weight conditions!