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My printer is built 300x300 with mk2a bed. I just took soft endstops off and measured the maximums that the carriages are able to travel.
The nozzle is about 45 mm (quick measurement) in front of the X rails so the bed needs to move forward to get the Y 300 mm coverage. That still leaves 5.6 mm (9.6mm - 4mm) to move off the build platform. Which is close but is what I have set up. It works most of the time, but if the filament curls too much it gets hung up.
Future Plan: I'm probably going to move the build plate rearward (I'll lose some of the build area.) and install some kind of purge box and nozzle cleaning, but it's good enough for now.
Edit: I measured the mk2a bed that I recieved from AliExpress, it measures 308 x 308. It seems that depending on the supplier the measurements can vary.