Can you get 5 reps at 360? more? Just calculate an estimated max and multiply it X .85. So lets say you can get 10 reps at 360..
360 x 10 x.0333 + 360 = estimated 1RM (479.88) round DOWN = 475
475 x .85 = 403.75 (round down, 400)
Now.. Test. Can you get 5 reps @ 400? If so, that's you're training max. You don't have to test, but since you don't know what your training max should be, I suggest you find your 5RM and make that your training max.
This is a tad overthinking. I've used the same training max for a long time on the squat and don't really use % for it. The 85% and 90% are guidelines, but meaningless once you understand the training max.
To recap.. Just calculate your last PR set on deadlift that was below 10 reps (don't calculate an estimated 1RM with more than 10 reps). Multiple that estimate by .85 and that will give you your 85% TM.