One centimetre here and there is really not inconsequential, use an electric measuring tool to be safe. No need to look for a power outlet, since it runs on batteries. Just get at it!
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Bosch Professional Laser Level GLL 3-80 (red laser, interior, working range: up to 30m, 4x battery, AA, in carrying case)
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Bosch Professional GLL 3-80 Self-Levelling Multi Line Laser Measuring Tool In Carry Case
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