I am still interested in what is being measured and that "slot". Is the folded bit relevant? Are you measuring across the folds or is it still a length that is being measured? Without knowing the physical aspects of what is being measured it is quite hard to see how to measure it.

I think that the variability is going to be quite easy. Assuming we are measuring a length, get a reference length a bit short of the target, say about 790mm. You don't have to know exactly what it is. Then use an electronic caliper or micrometer to measure the difference between it and your sample with them laid side by side with ends against a stop block (flat, machined...). Put the answers in a spreadsheet and normalise them or calculate the standard deviation.

The absolute is still hard, but can you not turn the absolute problem into a relative one. After all you want the parts to be 0.5 mm less than the slot. So, who made the slot? Does it exist and do you have it? If it doesn't exist yet, who is going to make it and can they provide you with a reference length. If you really do have to get a grip on the absolute measure then it is a pretty common technique, not just in metrology, to get an independent reference. I just suggest that it be referenced back to the slot.