Hi,
Above FL410 you are no longer in RVSM airspace, and the buffer between altitudes becomes 2,000ft. This means that valid altitudes are as follows (source):
Eastbound: 370 390 410 450
Westbound: 360 380 400 430
So even when flying east to west, you’ll never be planned at FL440 for example.
I’ve made some adjustments to the cruise altitude calculations for business jets. When planning an eastbound flight, assuming the aircraft is light enough (you can set 0 passengers/cargo to make sure), SimBrief should plan FL450 from what I can see.
Best regards,