How to calculate linear Y rod length for a larger replacement bed?

I have an Anet A8 which I want to convert to an aluminium frame printer like the AM8 (rebuild of Anet A8 parts):

This should give a much more stiff frame with respect to the acrylic frame to aid in a better print quality.

Since the bed (220 x 220 mm) has a slight warp, I want to use a larger build platform (200 mm x 300 mm) I have laying around. My plan is to use the 300 mm in the Y direction.

• Knowing the maximum outer distance of the current Y bearings to be 105 mm, is that too short for such a large bed?

• How do I calculate the length of the Y axis rods? (especially if you take another distance than 105 mm)

The overhang of the bed, assuming the bearings are in the center, equals (300-105)/2 = 97.5 mm on each side. So the distance from the leftmost bearing face (when bed is at y = 0 mm) to the center of the Y rods assembly equals 300 - 97.5 = 202.5 mm. Knowing this distance for the other side of the center to the right when y = 300 mm is the same, the minimum length of the rod will therefore be 2 x 202.5 = 405 mm which equals the addition of the bed length and the outer bearing distance 300 + 105 = 405 mm.
Please note this excludes extra length for e.g. a limit switch, and some extra space around the bed. Basically the extra length of the bed 300 - 220 = 80 mm needs to attributed to each side of the bed, so 40 mm on each side extra.
For a 220 x 220 mm bed, the minimum length would be 2 x (220 - (220 - 105)/2) = 325 mm. When I measure the rods between the acrylic flanges it measures about 365 mm, about 40 mm extra for clearance around the bed (20 mm on each side).