I would not say that is fine... sorry to disagee. Although fluctuation is normal, I have read over and over not to let the temp drop below 75F!

I too would move the bulb to the middle of the tank. You could also get a larger dome to hold the bulb, thus expanding the total area for which the bulb heats.

I really hope your window is North or South facing and that your tank does NOT get ANY DIRECT SUNLIGHT!
That turns tanks into ovens for snakes

I don't know if it's possible, but is there any way you could move around your room so the tank is not too near the window?