Measuring trickle vent area

Hi, we had exterior french windows fitted a couple of years ago, with one trickle vent on the right hand door. When the (very unhelpful) building control officer came round a while ago he said we needed another fitted but didn't say why. When I spoke to him a few days ago he said it's the area of the trickle vent that is important and that we need to have 8000mm2. Before we get him out for the final inspection, I want to check that the trickle vent we DO have is an adequate size, obviously if it's not there will just be more delays if he has to come out again. Is there any easy way to check this? He said you can't just measure it, so how could I find out? I don't know who made the doors so I can't ask them.

Thanks!

Of course you can measure area, it's width x height. One of these in each door should do the job.
http://www.mightonproducts.com/catalog/trickle-vent-brown-p-112.html

What was there before? Trickle vents are only compulsory if you had them before; if there were no vents in what came out, you donlt have to have them now! Same applies to fire escape on replacement windows; they are sensible but the criteria is the situation should be no worse than it was before.