I can only assume that door was open more than thirds of the way, otherwise you wouldn't have been able to see that much light coming out.
この場合、次のような言い方ができますよ。
ーI can only assume that door was open more than thirds of the way, otherwise you wouldn't have been able to see that much light coming out.
「ドアが3分の2以上開いているとしか考えられない、じゃないとあれだけの光がドアから出ているのを見れないだろう」
I can only assume ... を使って「…としか考えられない」と表現できます。
ご参考まで！
カナダ 
