Percentage Based Custom Prices

We already have custom prices per customer, which is really helpful. But it would be better if we could define their custom price as a percentage of the price tier they are at. This would make updating prices much easier as we would only need to update the price tier prices and all the custom prices will automatically recalculate. 

For us, we can't markup based on supplier costs because that price constantly has small fluctuations while our price list is constant, so we end up defining a custom price for each customer for each product. But every update to list price generates the corresponding need to update the custom prices since we generally give customers a percentage off the list price.