[b58.oscar]

I want new tires for my x drive m340 would 255/35/19 fronts and 285/35/19 rears fall under the 1% rule? i’m not sure how to calculate it.

[G22b58]

No. First you need to "compensate" a different tire width by using another profile number. Then, the nominal size and profile designation on the tire is no guaranty that the actual rolling circumference difference is within 1%.
What you need to now about the designation:
www/pp/dd:
www: tire width in mm
pp: profile in % (i.e. tire sidewall is www*pp/100 [mm])
dd: rim diameter in inch

Theoretical diameter would then be dd*25.4+2*(www*pp/100) [mm].

[HFW003]

Just use this, https://tiresize.com/comparison and no, it doesnt work.