Your COI is invalid, you should turn it in and you may not operate in certificated service. A custom engineered system is designed for a specific space requiring individual calculations for the extinguishing agent volume, flow rate, piping and similar factors for the space. A pre-engineered system is designed and tested to be suitable for installation without modification as a complete unit in a space of set volume, regardless of the specific design of the vessel. The attached sheet explains how to figure out gross volume of a space, and where to take the measurements. Accurate measurement and calculation is essential, because it determines how much extinguishing agent you'll need.
Nothing is exempted when determining gross volume of a space.
You can't deduct for engines, tanks or other machinery. No, we won't measure the space for you, but we will explain how to do it and answer any questions. As part of the installation inspection, we'll do some measurements to spot check your calculations. Make sure you hold on to your copy of your calculation work sheet so we can see how you arrived at your figures. If a professional did it for you, make sure you have their records and calculations available for us.