You're talking about 7-8% of your net worth. I'm probably the biggest moron on this forum. Believe me or not but for me almost everything goes into this passion. As long as I can pay my bills every month I feel safe to spend the rest. 10K on my bank account or 10 K for that new watch I'm saving fo ? . For me it's a decission I make in 0.2 seconds. I'll go for the watch. Some members on this forum are very wealty others are middel class (like me). Some guys can buy a watch after a good business deal they made, for me (and a lot of others) it's a matter of saving. I don't have a huge collection and I do not have the tourbillons or minute repeaters but I am very happy with the ones I have ! When your grail is the 5078 I might compare it with my grail which I bought in 2006. My 5070R.
It took me years before i decided to buy this watch. Why ? Because for me that was a lot of money. I had to sell almost every other watch I had in my collection, a collection which started in in the early 1990's. When I do not include my house I can tell you that it was a lot more than 7-8% of my net worth at that time. A LOT MORE !
Am I nuts ? maybe. But you can't imagine how much pleasure I got from that purchase.
Besides, I do ALWAYS look at the secondhand value of the watches I would like to buy. Because there could always happen something that I need to sell one or more watches. I almost calculate in watches. When I need 4-5 K I won't take 5 K from my account, I would sell a Rolex sub LOL.
So yes, I only buy watches with a very good resale value. Or I buy second hand. The only problem I have is that there a some watches out there which I would love to own but I can't because they don't fit in my ' good resale value' rule. One example is the Journe Chronometre Souverain. I would love it.
So, do I need to make money on a purchase ? NO that is not my goal but on the other hand I won't buy it when I know I will loose a lot on it, simply because I can't afford it, and there is nothing wrong with that.
Concerning your MR, If you can afford the loss you'll make if you would ever sell it (15-20% ??) than go for it. If you think loosing 20% on that amount is way too much, than don't buy it.
That is ofcourse my own opinio, others might have a very different way of thinking