This has probably been said a hundred times before but here goes.
Simply weigh your jiffy pellet/rockwool before first soaking and then weigh it again after soaking. The difference will be the volume of the water. 1g = 1ml. For instance, my jiffy pellets weigh 7g dry and 57g wet so thats 50ml of water, dry rockwool (40mm) 5g, wet 40g =35ml of water. This is only helpful if you have scales capable of weighing grams. So all you have to do is weigh the item and add the desired amount of water. PrecisionGardening.LOL
“Why is marijuana against the law? It grows naturally upon our planet. Doesn't the idea of making nature against the law seem to you a bit . . . unnatural?”
Yeah I like this method, show 'em the love 'ey.