Much better to move it rather than cut it back that hard. Cutting back by half is not considered 'trimming" -- in fact, it is not recommended to take any tree back by more than 20-25% at any given time. It is far too much of a stressor on the plant. And it is never a good plan to limit a tree to less than half its normal mature size - they are not shrubs - and even less of a good idea with a Japanese maple for a whole host of reasons. 12' is still pretty small for a 'Bloodgood' so perhaps a better location or a more appropritae choice is in order......unless you are willing to wait until it grows sufficiently taller that you can limb it up so the canopy is above the window. Wait until it is dormant to relocate or if too close to cold weather in your climate, do so in early spring before it leafs out.