Must admit I don't allow for the weight of the scale, but then a bow generally looses a few pounds over the first few months of shooting and I'd rather be a few pounds over than under.
I don't get too hung up on and tend to say things like 'It's a good 40#'
There's a fair bit of inaccuracy in measurement anyway, you only have to winch it back and wait a minute to see the pounds drop.
Also I don't always bother to zero the ruler to the back surface of the bow.
I expect the general accuracy is probbly plus or minus a couple of pounds at best, and probably more than that.
Del