Lady Heath, I'm afraid the answers to your questions will often be of the "it all depends on....." type.
As pilots, we tend to be rather conservative in spite of what might get written on PPRUNE. This is probably because we prefer the engines to stop after landing, rather than before, so fuel consumptions tend to always be rounded up.
To try and give some answers:
For 90hp and 120 hp aeroplanes a sensible cruise figure might be 75%, but the Pilots Operating Handbook (POH) for the individual aeroplane will often give fuel consumption figures for other settings. There will typically be tables of fuel consumption and Air Speed versus Manifold Pressure and rpm for different cruise altitudes and temperatures in the POH. The POH will also give the climb figures (normally only at gross weight). Incidently, 90hp and 120 hp aeroplanes would normally climb at full power.
These figures are for a new engine and aeroplane with a perfect pilot running the engine at the optimum mixture setting.
For taxi it is usual to allow a fixed amount. I allow 1 U/S gal in a light aircraft.
For a flight under instrument flight rules (IFR) we add 45 mins fuel for holding, plus enough fuel for a diversion to the alternate airfield. I also add 15 mins fuel for the approach and then add the whole lot up and add another 5% for contingency.
For a flight under Visual Flight Rules in good weather in a small light aircraft with fairly limited range (only 12 UK gals) I will sometimes fly the return leg of a flight knowing that I will only have 20 mins usable fuel left when overhead my home airfield. However, I have an alternate airfield only 5 mins flying time away. I am prepared to do this in the above circumstances, but others might not.
Good luck with your project !