I just use the dodgy bathroom scales. I weigh myself before, write down my weight and then go downstairs, grab the malt, which I have put into a bucket, half full by eye, maybe have a drink or something to eat, go back upstairs and stand on the scales with the bucket balanced on my head (so I dont throw of the balance). I then write down my new weight and subtract the old weight from this, this gives me the weight of the grain. I repeat this process 5 or 6 times till I get the required quantity.
I have found this method much better than my previous one. I only had envelope scales that measured upto 50g, so I would weigh out 50g of malt, then spread it all out on some graph paper so that no grains were stacked (so the thickness was 1 grain thick) and count the number of squares that 50g of malt took up. Once I knew the area that 50g took up I could multiply out by my required quantity. I found that my driveway was the only area large enough to spread out my 5kg of grain spread to 1 grain thickness, so on it I marked out a 5cm grid (3g accuracy!). It was then a matter of spreading out the malt until enough squares in the grid were filled. It was quite a simple method but I had to find another way because a flock of cockies started hanging out in the trees near the house and would descend on the spread out grain while I was carefully eyeing along the ground to make sure the layer was only one grain thick. So I started doing it at night, but the cockies still came but they couldnt see in the dark, so now Im missing the top of my left ear and my right thumb.
