Letsby, there is frequently confusion about power required for transitions and thrust margins expressed as a percentage.
A HOGE plus 10% thrust margin is basically derived from loading the aircraft up until it only just has enough power to hover OGE and then reducing the AUM by 10% - this gives you an AUM that will allow you to manoeuvre whilst in the OGE hover (some flight manuals specify actual rates of climb or the ability to cope with light turbulence).
The 10% figure that appears in many teaching guides is in fact 10% torque in hand - it is basically a safety measure to highlight that for a normal transition to forward flight (ignoring advanced cushion creep techniques and the like) you need to have available a margin of 10% torque over and above what you are actually pulling in the hover at normal hover height.
So if you are pulling 90% torque in the hover and your maximum available is 100% then you are OK. If your max available (due to other engine limitations like temperature) is reduced to less than 100% when you are using 90% in the hover, you will either need to use a more advanced transition technique or offload pax/freight etc.
Hope this is clearer.