Hello... my 2 cents:
I read that inflating a balloon was not exactly accurate because when you blow into it, some pressure builds, and in fact due to the ballon pressure, the volume in it is a little smaller that the V
you have in your lungs. So my measure method was:
Inflate and deflate a couple of times a ballon, to free it from its presure and then fill it as much as i could in a single breath. Prepare a mettalic bowl full of water with a volume rule at one side (those are used to cook). See what the volume is without the ballon in, submerge the ballon ENTIRELY and measure the volume now. The difference (i.e: the volume of water displaced by the ballon) is the VC you have in your lungs.
Accuracy: With this method, I measured myself 6.5 liters of VC. Went to a diving medical certificate... and they measured 6.7 liters... so It's really acurate!
Regards,
Oscar