How To Calculate What To Charge For a Maintenance/Service Contract

What formulas do you use to figure out a maintenance / service agreement? This would be for all or just 1 of the following - card access, video cameras, alarm systems and fire systems. In some cases it would be equipment only, labor only, both and or preventive maintenance. Would you give an option for software and firmware upgrades? Option for clenaing cameras? Do come up with a certain price on a new sale. how to determine if they want to add a year or 2 later? what if you didnt install the equipment.

Any help would be appreciated. :)

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