That is correct, I wrote that myself and forgot to edit the OP here! I'll do that in a second.
Sin impacting the level range was disproven, through extensive testing using very different methods (by myself, and also by someone else with a friend on PC). Post number 12 in this thread
It is surprisingly just a fixed level range based on the invader's level after all. It's minus "50 + 20%".
edit: Whoops, forgot to address the rounding. You're exactly right about the rounding and calculation
. Level 63 Darkmoon can invade level 1, but Level 64 can't.
For those who may be unaware, the way the level range rounding works is that the decimal place is always truncated (or rounded down), before the rest of the calculation is made.
For example, a level 49 player trying to find out their range uses +/- 14, because the "10 + 10%" of the formula is actually just "10 + 4", not "10 + 4.9".
So in the case you mentioned, 20% of 64 is 12.8 That gets rounded down to 12, and then 50 is added to it. You get 62, and 64 - 62 = 2.
I just edited this into the 'general' section of the OP as well.