Using Zen Cart v1.3.7 full-fileset 12302006

The problem I have is, I have to work with FIXED Muliple Quantities at FIXED Prices per Multiple Quantity from a manufacturers list.

I've tried this in the ' Products Price Manager' page using the 'Discount Qty Applies to Mixed Attributes' section in various settings, but the end price just will not tally with the price list I have to work with, by either rounding up or down to the nearest price, but not the exact price I have to work with.

I've tried altering all instances in the database figures for 'prices' from
(15,4) NOT NULL default '0.0000',
(15,9) NOT NULL default '0.000000000',
Hoping that the increase in decimal places will correct the price, calculating the individuaul price of an item from the quantity, but it seems to make no difference

What other areas in the database need to be reset to calculate down to more decimal places?

Just from working with one of many price lists
20 items = 66.45
30 items = 76.40
40 items = 86.35
50 items = 96.30
60 items = 106.25
70 items = 116.20
and so on

Is there an easier or different way of doing this.