Couple of things to note here. Firstly, please mention which guide you led you astray so we can have a look and make sure the info there is accurate. iFixit certainly doesn't want you or anyone else's devices to inadvertently come to harm due to misinformation.
Secondly, although screw charts are helpful they are no substitute for common sense and good organization. Investing in a good magnetic mat, screw organizing tray or literally a piece of tape sticky side up has saved my butt on more than one occasion. If a screw doesn't feel like it goes in a place, it probably doesn't. Lay out the screws near their cowlings in the same pattern you pulled them out of the device.
Thirdly I don't have a screw diagram for these that includes measurements but you might find the [https://www.mobilesentrix.com/iphone8plus-screwbox|Mobile Sentrix screwbox page] helpful. No screw sizes, but it does have pictures and locations of the screws.
You also mention both the 8 Plus and the XS Max in your question so I'm hoping that's the correct one. Also. If you're talking about the "long screw" for the cowling over the display, battery and other connectors, it goes in the screw hole closest to the speaker.