Topping and power assurance check
Maybe I can help a little as maint engineer - eventhough not on Lynx but 212/412, EC 135, A-109.. (and some big jets too)
On PT6T engined 212/412, topping check means spinning one engine to the Ng RPM limit - actually running it to the limit. The purpose is to check if engine RPM limiter and condition of engine actually allows the engine to achieve max. Ng RPM. If not - adjustment of FCU or/and comp. wash is required. In some conditions (dense air) it is difficult to achieve, as Torque limit might come first. Since this is streching the engine to the RPM limit, it is normally done only twice a year - under current PWC maint. schedule. Normally, it is done on the groud with ballasted helicopter to prevent take off. It can be checked also by putting a "wedge" under mechanical stop on FCU and checking this limit with reduced RPM. (called part power check)
Power assurance check is merely an indication if engine is delivering minimum advertised power, thus validating helicopter performance charts. Following FM, it should be done daily (I wonder how many of you are really doing it).
The point is, as you know, to set a certain torque output and, depending on athmospheric conditions, this torque shall be achieved with certain Ng and ITT values. Graph provides you with max. Ng and Max ITT for conditions given. The difference between actual ITT and graph ITT is called ITT margin (and same for Ng margin) and tells you how much wear (and where in the engine - is it compressor or turbine) you have and which limit you might hit first in high altitude/hot OAT. As engine gets older, ITT and Ng margins are of course gettting lower. Typically, after compressor wash, Ng margin will improve. These numbers are also tracked as "engine trend monitoring" and can sometimes give a clue on engine trouble.
Honestly, after 20 years in maintenence I can say that readouts from "steam gauges" are so notoriously unreliable, that trend monitoring them is more guesswork than actual monitoring. Things are much better with FADEC engines and more accurate instruments and readings. The only real check I would bet on with "classic" engines is to perform single engine climb - taking your charts for single engine climb and actually doing it on single engine with max cont. power on a calm day with exact weight - initially it will be torque limited, but with altitude, it will switch to Ng or maybe ITT limit. With stopwatch and accurate climb profile, you determine your rate of climb at certain altitudes (so you can accurately enter in your single engine climb graph)and that gives you a good idea on your helicopter performance - the trick is, that engine power itself might not be everything -it is also flight controls/rotor RPM setting, blade condition etc. that determines your performance.
My five cents worth.
hoistop