I realise it's a matter of personal taste, but about how much Soy sauce should I be using, in a portion of mushroom stir fry?

Splash a couple of shakes in and then taste. My motto in the kitchen when seasoning is taste, taste, taste, then taste again. Remember, you can adjust under seasoning but if you over season you're basically f****d!

I can never understand why a lot of people put something on the table without having tasted it first. I'm not saying you do that but it's amazing how many people do. Sort of like a game of culinary Russian Roulette!

Dark or light?

3-4 tablespoons.

Light.

It's called Sood's Fine Foods Light Superior Soy Sauce.

Less than 3-4 tablespoons then, as it'll be saltier than dark.

3 tablespoons for 1 portion? That sounds a lot to me. Is that brand less salty than others? I do however bow to your superior knowledge looking at your username! I'm assuming it is short for Kikkoman?

Light soy is for 'salt' dark is for 'colour'.
Rule of thumb so to speak.

That's about a third of a bottle ! Nah rather less than that I think of light soy.

Stick in a couple of teaspoons to start with then taste. Add more teaspoon by teaspoon until get what you want. This way you will know for next time.

I'd go with this

eh? really!?

A tablespoon is 18ml, so four would be 74ml, which makes quite a dent in a standard size 150ml or even 250ml bottle of soy.

That is an awful lot of soy sauce !

I never taste until it's on my plate. So far so good.
My nose and intuition tells me how much seasoning. I turns out allright.

You have much better intuition than me then LCG!!