Contingency in large bell projects In my commercial world I wouldn't include less than 33% contingency, often 50% and sometimes 100%. The reason being that you cannot possibly think of everything and cost for it - or, if you did, you'd come up with the higher figure anyway. It's just not worth spending the time thinking about it - just accept you will almost certainly use contingency.
The thing with bell projects is that, in the scheme of things, they are small & simple (may not seem so to many involved but they are a well trodden path with more certainty then the stuff I deal with). So less contingency is probably OK although I feel 10% is light.