Calculating Ash

Is there a way to calculate the ash in a food when it's not listed on a food? 

I'm working on a chart of the level of protein, fat, fibre, moisture, ash and carbs in the different wet foods we sell so that we can help our customers more quickly and accurately. There are a couple of brands where the ash content isn't listed and I can't find anything out online. Short of contacting all of the manufacturers, is there any other way to calculate the ash content?

Here is a link - it's an excel file - for DMB, including ASH calculator, from Catcentric's website: http://catcentric.org/wp-content/uploads/2011/10/DMB-Calculator.xlsx

http://catcentric.org/

Thanks so much!