When you're measuring D8, are you measuring from one pin to the other, or are you measuring from the positive pin to the ground on your RAMPS board/PSU?
RAMPS provides 12V ALWAYS on one pin, and switches the connection on the ground (using a MOSFET).
Not all MOSFETs are created equal, and the ones shipped by default on the RAMPS boards (STP55NF06L) really aren't good for high current loads.
If you get a similar voltage across D8, then I'd look at replacing the MOSFET for the bed. That said, 0.6V loss in the MOSFET is not huge, and I suspect that even with a good MOSFET like the IRLB8743's or an FDP8870 you're not going to see much lower drop than that.
If you suspect the wiring to the bed, while the bed is heating measure the voltage from one end of a wire to the other. eg: One probe on the bed end, one probe on the RAMPS end. If there's any appreciable drop on both wires, you might consider replacing it. I doubt this is an issue in your setup if you're only losing ~0.6V.
Regarding wire to the bed: You want reasonably flexible wire that is capable of carrying a good current. While thickness (gauge) relates to this, you can get types of 18 gauge wire that will carry more current than other types of 14 gauge wire. eg: 18 gauge RC hobby wire will do heaps better than 14 gauge 240V mains wire.
I usually recommend you look at RC Hobby places, as RC vehicles (of all sorts) tend to use very high currents, and the wire they sell tends to be very good at high current and is usually very flexible. They also tend to have decent supply chains so getting your hands on it is not an issue.
Note: The bed draws quite a lot of current. A 200x200mm square PCB bed draws 9A->12A at ~12V (usually more likely to be on the 12A side). Compare this to the hot end (extruder heater) which draws between 1.75A (6.8 ohm resistor) -> 3.4A (40W cartridge heater).
RAMPS provides 12V ALWAYS on one pin, and switches the connection on the ground (using a MOSFET).
Not all MOSFETs are created equal, and the ones shipped by default on the RAMPS boards (STP55NF06L) really aren't good for high current loads.
If you get a similar voltage across D8, then I'd look at replacing the MOSFET for the bed. That said, 0.6V loss in the MOSFET is not huge, and I suspect that even with a good MOSFET like the IRLB8743's or an FDP8870 you're not going to see much lower drop than that.
If you suspect the wiring to the bed, while the bed is heating measure the voltage from one end of a wire to the other. eg: One probe on the bed end, one probe on the RAMPS end. If there's any appreciable drop on both wires, you might consider replacing it. I doubt this is an issue in your setup if you're only losing ~0.6V.
Regarding wire to the bed: You want reasonably flexible wire that is capable of carrying a good current. While thickness (gauge) relates to this, you can get types of 18 gauge wire that will carry more current than other types of 14 gauge wire. eg: 18 gauge RC hobby wire will do heaps better than 14 gauge 240V mains wire.
I usually recommend you look at RC Hobby places, as RC vehicles (of all sorts) tend to use very high currents, and the wire they sell tends to be very good at high current and is usually very flexible. They also tend to have decent supply chains so getting your hands on it is not an issue.
Note: The bed draws quite a lot of current. A 200x200mm square PCB bed draws 9A->12A at ~12V (usually more likely to be on the 12A side). Compare this to the hot end (extruder heater) which draws between 1.75A (6.8 ohm resistor) -> 3.4A (40W cartridge heater).