So 'calibrate' what you measure against what you see.
When you inspect, total up the capped and uncapped stores you see. Estimate roughly how full each frame is. For example, an end frame with nothing on the outside, and a well filled but incomplete other side, would score about 45%.
Go through all the frames and total up the %.
A total of 525% would mean you had about 5 and a quarter frames-worth of stores in the hive.
On my personal basis of 2.5kg for a 100% full National deep frame's content, 5.25 frames would be just over 13kg of stores. (You'd hope for 20kg before winter.)
Now if you subtract that 13.125kg from the total weight measured (but see below) that will give an estimate of the weight of stuff that isn't honey (wood, etc) in the hive.
If you do this exercise a few times and average those estimated non-honey weights, you'll get a fair estimate of the weight of that particular empty hive. After that, you can get a good idea of the stores by weighing and subtracting the hive weight.
Doing the weighing.
Attach a simple hook (that your scale can attach to) onto each side of the hive, half way back. Best on the floor, since the brood usually sticks to the floor...
Weigh both sides (you only need to lift it by a fraction of an inch) and then add the two weights together to get the hive's total gross weight.
Weighing from the back alone is not enough.