I think youíre confusing calibration with the suggested carrier volumes on the label. Manufacturers canít test all possible carrier volumes, so they test a range of volumes and list those ranges on their labels. For example, the Celsius herbicide label recommends that it be applied using a minimum of 25 gal/A. The Trimec Classis label uses older language. It says to apply 3.25 to 4.0 pts of product per acre with spray volumes of 20 to 260 gal/A (or 1.2 to 1.5 fl oz of product in 0.5 to 6 gal of water per 1000 ft2). All that means is that they have tested the product at carrier volumes in that range (20 to 260 GPA) and they have gotten good results.
The product application rate (using Trimec Classic as an example) is the 3.25 to 4.0 pts of product per acre. So, it doesnít matter what your carrier volume is, you only use 3.25 to 4.0 pints of Trimec Classic per acre.
Where the carrier volume comes in is that it helps you to know how much product to load into your tank. Your carrier volume tells you how much area a particular amount of mix will cover and the rate tells you how much product to put in the mix.
So (using your example), if youíre spraying at 6.25 gal/M, that comes out to 272 gal/A (maybe a little high, but this is just an example). Letís use the 4.0 pts/A Trimec rate to use a whole number. You would load into your tank 4 pints for every 272 gal of total mixture. If youíre only loading up 50 gallons, you can use the ratio (50/272 = x/4) or you can use the area figure (50/272 acres * 4 pts).
This thread has shown that there are lots of different application volumes to use and lots of reasons why to use them. But, the amount of product you put in the tank will always be calculated on how much product will cover the area covered by your carrier volume.