There really is no perfect dry time, as all plants dry different, 1st from how dense the bud is, how many stems, how much trim, growth, all that.
Then there's environment. If that temp is true at 12.5c, then you are looking at maybe double the time it would take if your dried it at what most research and stuff out there on drying is usually in between 17-20c. Your RH is grand. A lower temp is of course better, but it will make it much longer.
You have a number of ways by telling
So, what you want to be aiming for is 15-20% moisture left in the bud. So, to know this, you get a stem, and bend it, it should snap but be still whole, (not break off) and a simple grind will put you on the right course, now part of your plant will dry quicker, as you are drying whole, (its the best way, but it is the hardest when drying) as you have to take parts of and put them to the cure when the rest of drying, (its why most ppl just dry node by node and trim it back to bud, which is wrong) well, it's not wrong, but, the way you are doing it is the right way.
As for weight, usually for me it's 1-3 1/4 of the wet weight you should be left with, but again, it depends how dense your bud is. And you won't be able to tell by plant weight, take a bud and weight that, when you get to or near 1-4 or less/more depending how its grown will help you determine when you near ready, but neither way will give you a straight answer, as a stem or bud dried whole on a plant will dry quicker the more it's exposed to the outside air.
A good way to tell, is if you grind it also. Tbh, you'll pick up your own way of drying, yes, you can follow a process, but as I said, no dry will never be the same, nor can you follow and instructions and find an exact path to success. Best to take it in stages. When the outside nodes are dry, or after a good few days of the plant taking in the plants natural sugars and that, and the leafs are lifeless you can start to trim some and break her up to get a more uniformed dry.
Feel free to DM me regarding any of this, as drying differs massively to bud quality, how you harvested, (some ppl water an hour before harvest to prolong the drying time) will all have an affect on how your bud dries.
Good luck pal.