Interestingly enough, the refund calculator disappeared from the Global Blue website. ( since April 2020 maybe).

In my experience, expect Global Blue to take 50% commission of your refund, and to skim off further from the forex difference. I always asked to be paid back in local currency, something that short changes them. Since no one is shopping, I personally home Global Blue, Premier Tax free don't survive - and a more reasonable scheme exists that doesn't skim so much from people doing shopping.

I have bought high priced items that totally bypass Gliobal Blue, it is possible, but you need to do the legwork.