Most road bikes have small screw adjusters that determine how far back the hub can go. If you have those then push the hub as far back as it can go and tighten there. Yes it does affect the chain tension to some degree and also affects the alignment of the wheel in the frame. If you don't have the adjusters you still should push it back as far as you can but make sure the wheel is straight in the frame before you tighten the lock.

I've no idea why the dropouts are horizontal. As someone who is pretty good at ripping back wheels out of them I sometimes wish they weren't.