simonc m guessing 15g is too little for the 20g vst. Should I get the 15g vst or could I go with the 18? Are there are recommendations like IMS?
I dose 15g in an IMS E&B Lab B702TFH22, which 22mm deep and is rated for 14/16 gram (so I assume it is the IMS equivalent of the VST 15g basket).
15g of coffee leaves plenty of head space in this basket. The amount you can dose depends on the coffee of course, but with every coffee that I have used I could easily dose more than 15g in this basket - surely 16g but I think even more. I therefore think that if you want to dose 15g you should get a 15g basket, not an 18g one.
If you lower the dose from 19 to 15 grams you will definitely have to grind finer to get a similar flow rate through the bed of coffee. I believe that’s a result of the thinner puck, not the size of the basket.