Thanks for the information and advice. A 55/45 split looks like a reasonable target and achieving that with similar brake spec front and rear plus some form of adjustment (balance bar, valve) for fine tuning makes sense.
Coming back to the original question about Centre of Gravity Height the calculations show that whilst it has an effect (the lower the CoG the less the weight transfer and the more rear brake bias required), its not as critical as i had thought it would be.
The results show that the "Optimum" braking ratio changes from 52.8%/47.2% (Front/Rear) at 550mm to 48.2%/51.8% at 350mm. The "Optimum" allows no tolerance for brake performance variation so adding in some margin for that, 55%/45% seems sensible.
But, I shall build my car with the brakes I have and look at an upgrade for the rears later, after I've had a chance to drive it.

Happy Christmas,
Keith