The technique I teach for manual control of an engine in the C++ is to set 40% Tq with trim and just leave it there. As long as there is torque on the other (good) engine the RRPM will be governed at 107%. There should be no need to adjust the power on the manual engine at any stage of flight, unless you're heavy and landing to a hover in which case you just trim up as required to keep the governed engine from driving up to an OEI limit. For a running landing with manual engine at 40% Tq there is no risk of over-speeding the rotor when lowering collective after touch-down.
When landing to a hover with an engine in manual and high power setting it is very important to co-ordinate the lowering of the collective with the reduction in engine trim otherwise you can drive the governed engine into an OEI limit (if trim down power too quick), or overspeed the rotor (if you lower the collective too quick without reducing manual power). With some prior thought and a little practice it is very easy to operate the C++ with an engine in manual.
Double DECU failure and operating with both engines in manual is also straight forward if you lead with up collective prior to increasing power, and you lead with trim down before lowering collective. There is no rotor speed governing possible with both engines in manual control. This is something not often practiced with students as the likelihood of it ever happening is very remote.