I'm thinking it must be something in the high 300s... A quant score of 18 is a pretty low percentile, so you need to focus more on quant than on verbal.

You can check out a score estimator here: https://www.800score.com/score2.php

However, it's no longer up to date. If I put in my quant 50 and verbal 47, I get a score of 790. However, on the real thing I received a 770. This is why I'd advise you to subtract slightly from what you obtain by using this tool, because percentile rankings have gone down in the past few years because of people getting higher and higher scores.