That’s an interesting question. Most folk want to increase travel. In which case the stock answer for oem is “I’m pretty sure you wont know if they are adjustable til youve taken them apart (to remove spacers)”. Some / most oem dont have spacers. In the ones I’ve had spacers seem to be either 15 or 20mm.
As you’re reducing travel I’d probably be happy to add spacers to reduce travel (its daft / impossible to increase it beyond it’s limits). In your case all its going to do is force it so sit lower on the stanchions.
btw – I’ve only gone the other way – removing spacers. If you die – it’s your own fault!