Using a hat stretcher

[Old Fogey UK]

I bought an "antique" wooden hat stretcher on eBay a while back but I'm not sure how to use it effectively.
It's shaped like two halves of a doughnut with a grub screw handle connecting the halves. There's also a little brass strip in the gap between the halves that's calibrated in hat sizes. It's really heavy and has a slight taper to it.
Do I just put the hat on it and open it up to the appropriate size ? Do I wet the hat ? How ? Do I stretch the hat in gradual stages ? How long do I have to leave it on the stretcher ?
I have a 7 1/4 or sometimes 7 3/8 head. does this gadget mean I can buy 7 and 7 1/8 hats and open them up ?
Or am I wasting my time with it and should I send a hat to an expert to stretch it ?
I'd appreciate some advice from those in the know, please.

 

[SHARPETOYS]

Only one...

I have had my best results only going up one hat size.

 

[Pilgrim]

I have a similar stretcher. For minor adjustments, just put it in the hat, turn the screw and put appropriate tension on it. You have to decide what's appropriate, but remember - you can always do another stretch if it's not enough.

My experience is that for just fine-tuning, you will need to leave the stretcher in 2-3 days, and a week is better.

If you want to take a hat larger, I'm comfortable with increasing 1/8 size, but after that I have reservations. Do it a bit at a time, allow a few days to a week per adjustment, and moisten the sweatband slightly when you change adjustments. It may take 2-3 weeks to get it where you want it.

I imagine that others have different techniques, and will share them.