When i fed my greyhound complete feed i was feeding almost 1/2 what the packet says. All complete feeds seem to recommend too much hence why so many folk have overweight dogs. As you've already mentioned someone fed there's 1/4 less other 1/3 less. Like humans dogs will have different metabolisms.

I now have two Saluki crosses [both 18 kg]; the 2 yrs has a 1/3 more than the 4 yrs old as she is a more energetic dog... and just needs a bit more. I weigh them every couple of months and weigh their food each meal [although i'm now feeding a wet diet]. I do think weighing a dogs food is important - a mere oz of extra meat will result in the 4 yr putting on weight although the 2 yr can have a bit more and not appear to put weight on.

Just for your interest quantities for a wet diet are easier to calculate. The guide for decades has been; for meat feed 2 % of body weight + 1/2 of biscuit/mixer.
18 kgs = 40 lbs. so feed 0.8 lbs meat per day = 13 oz.

I give 6 oz meat + 3 oz mixer per meal for the 4 yr. 8oz meat + 4 oz mixer to the 2 yr as she needs a bit extra. This guide i've found is spot on. Even tinned food recommends far too much.

Hope this helps.