Don't worry about what the ISP marketing claims to have sold you, just worry about what your tests reveal .
Run a speed test with no QoS enabled, and pick the values that seem to beset represent your maximum steady state transfer rates for upload and download. Use those values as the maximum, and then apply your percentages on those tested values.
For example, when I run the DSLReports speed test on my link, I get around 43Mbps for download and a nasty C rated bufferbloat result with peaks in the hundreds of ms. I applied the recommended 85% QoS overhead and come to around 29Mbps and enable cake. Vastly improved, but bufferbloat is at A level with average bloat 20ms above idle. Dropping down to 28500kpbs results in A+ bufferbloat and average bloat within 5ms of idle. Perfect .
And thank goodness we don't need to wait 5 minutes between QoS adjustments like with the TrendMicro scripts!!
Awesome, try taking it up to 95% and see if you still get As
Note, I am just curious...