Hi Ashdaman,
First of all good luck with your Class 1 medical hope it all goes well for you. I had symtoms of asthma (wheezing barking coughs) as a child, I am of similar height and age to you and my peak flow is the same and I have had no problems with my Class 1 medical. My peak flow on the day reached 600, it must have been my adrenaline kicking in and just blew as hard as I can. You will be asked to take a spirometry test which will check your lung compacity, only tip I will give is to blow out as hard as you can (til you feel like you are going to fall over) as they measure how much air you can breath in. They require 70% lung capacity or more. Also bring any inhalers you use with you on the day and be upfront about everything it can only bit you in the rear if you don't
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Best of Luck