There is another factor at play here. If you are loosing 80% of the weight of trimmed buds in drying they were way too wet when they were harvested. Late flowering should be in humidity of low 40s or 30s if you can get it that low. This is too prevent bud mould. I think over 50% lost in drying is an indication that there was too much moisture in the buds at harvest.
So the answer to the question: how much weight will I lose in drying? can only be answered with the question: how wet are your buds?
The crazy thing about this thread is there is no good reason to weigh until dry anyway. A pound of fish wrapped in 10 pounds of paper is still a pound of fish.