Controlling humidity in an already humid environment.

I got my first incubator last week and set my first 'test' batch on Saturday. Nine that had been in the fridge for a day or 2 and 4 kept at room temperature. All 13 are developing veins so I'm thinking I'm doing something right. (So far!)

My concern is the humidity. How much does the room humidity effect that inside the incubator? I live on the NE coast of Scotland and it's very damp here, even in summer what with the sea fog!
The room is around 18°c and 78% humidity at the moment. I've been moving my weather station from room to room all day to see what they all are!

I am keeping the incubator set at 37.5°c as it's the forced air kind and it has an automatic turner so I only need to open it to candle the eggs.
I added 300ml of warm water at the start as it says in the instructions and have added 100ml twice since.

Until my hygrometer arrives I'm just guessing at the moment!

Any tips for a newbie?