This is a direct quote copy of the text from the pat Kelly site:
Caveats: Running an engine at the upper limit of the DCR range requires that the engine be well built, with the correct quench distance, and kept cool (170º). Hot intake air and hot coolant are an inducement to detonation. The average driver has no control over this when driving in traffic. If you anticipate hot conditions, pulling some timing out might be needed. A good cooling system is wise. Staying below 8.25 DCR is probably best for trouble free motoring.
>>Unless you have actually measured the engine (CCed the chambers and pistons in the bores), these calculations are estimations, at best. Treat them as such. The published volumes for heads and pistons can, and do, vary (crankshafts and rods, too). It is best to err on the low side. When contemplating an engine of around 8.4 DCR or higher, measurments are essential, or you could be building another motor.
The underlined part is mine.