Geez. I am thinking about MrDEB's projects while I'm in the shower!
I realized something. Somewhere along the line, we're changed projects and we're talking about 5 volt LCDs and therefore presumably not the POS 18F25K20 with its 3.3v limitation.
That means the diode in MrDEB's schematic isn't there for the voltage drop (to ensure battery voltage doesn't go too high) but for reverse polarity protection.
I suppose MrDEB plans on a Shotkey diode even though that's not what the schematic shows. I don't know what the intended power supply is for these "calculators" but most likely it's batteries and a 5 volt regulator. In any case for a battery powered circuit, a diode isn't the best reverse polarity protection because of the voltage drop and the power dissipation loss of the diode.
I have settled on using a P channel MOSFET for reverse polarity protection as described in my article
Power Protection Circuits. With appropriate MOSFET selection, the voltage drop is less than that even of a Shotkey, and I×R losses are vanishingly small. On a surface-mount board, an SOT-23 package won't take up any more room than a diode.
If you want to use this idea for higher voltages, a zener diode and a resistor should be added to protect the gate from excessive voltage as mentioned in the commenrs to the article. For 5 volt supplies, only the MOSFET is required.