I don't think you will find a simple formula that connects all these cases together. If I were doing the pricing, I would just use linear interpolation based on the values in your table. For example, if you have a comic priced at 70 for NM-, then the adjustment for VF should be half way between 0.4615 and 0.4533, since 70 is half way between 65 and 75.
I suppose there is a way to automate this computation in Excel.
That's assuming I understand your problem correctly, which I may not.