To do spacers correctly they should be hub centric spacers. Several years ago had a set of hub centric 10mm spacers made for the rear wheels of which I had three different size wheel/tire combos running in the rear and the 10mm spacers was run on all the wheel/tire combos and not have tire rub anywhere
My only regret with those spacers was I didn't have them black anodized.
Last year installed a 17x9 wheel which I could only get in a 12 mm offset. A larger 275/40x17 tire was going on front and rear. The 10 mm spacers where moved to the front wheels to allow proper fit. A 21 mm spacer was then made for the rear wheel fitment and this time had it black anodized. Both the old 10 mm and the new 21 mm spacers are hub centric and do an excellent job of maintaining the hub centric fit.
The easiest place to see, understand, hub centric is look at the front rotors to see the machined area of the rotor that is in contact with the center bore of the wheel.
A spacer moves the wheel outboard and a disconnect of that small in height and lightly tapered area goes away. A hub centric spacer will be machined on both sides to produce the fit to maintain hub centric. It actual has a male and female side. My guess is if a non hub centric spacer is used the max thickness would be much less that 1/4" before hub centric is gone.
1 mm = .0394". 10 mm is a little more than 3/8". Not that thick but necessitates hub centric.
Now with ARP rear wheel studs, Moser axles and the hub centric 21 spacer I have no fear of not being safe and reliable.
Be aware that wheel stud length/lug nut engagement is a huge factor in safety. 100% thread engagement is preferable in my book. In drag racing they wanted to see the studs sticking out of the nuts. Just keep the stud length factor as part of a wheel spacers install.
Everything about this company was first class. The anodizing is done "out sourced" and adds time to the order production and a few more dollars to the cost. Worth the wait. https://www.motorsport-tech.com/
The thickness of my spacers no where near represent anything close to what is needed with another Monte. My spacers are very unique to only my fitment. For your install I could only hazard a guess for spacer thickness. That wouldn't help you.
I was fortunate enough to have alum "junk" spacers of different thicknesses laying around so I could arrive at that odd 21mm spacer thickness. Two years later and hindsight now says they could have been 22 mm, just a silly one mm larger could have been done. A frame touch up of black semi gloss in the RR tire to frame area could be fixed by another .0394" at that location.