How to wire LED strips into my display cabinet?
I would bench test the layout first. I assume you can cut a 100 cm strip into three sections of 40 cm, 40 cm, and 20 cm. Wire up one 40 cm strip. Check result. Wire up one 20 cm strip. Check result. When you get a good result add another strip in parallel across 12 V. Check result. This way you would cut the minimum length necessary for a good result and as long as you don't exceed the rated amps on the power supply it should scale to six shelves with good result. This will require whatever amount of wire necessary to run for the geometry of your setup. The amp draw will go up for each strip added in parallel across 12 volts so be careful not to exceed amp rating of your power supply with a margin of safety. A typical multimeter can measure 10 amps on a fused input so I would measure the amp draw if possible when you get good results in a particular strip length.
This reference describes voltage drop on typical 12V LED strip light:
http://lednique.com/technology-basics/flexible-led-strip-light/
This video describes custom LED strips that solve the voltage drop and lack of uniform lighting problem using a smart variable resistor in each LED:
https://youtu.be/FrRJoauDD5A?t=154
This article describes driving LED strips with current versus voltage source:
https://www.ledsupply.com/blog/constant-current-led-drivers-vs-constant-voltage-led-drivers/