Normally, shaping restricts allowable bandwidth. We can shape traffic at 75% of a link's bandwidth, which is rather pointless unless we're trying to guarantee 25% for other traffic. Even then, I would consider CBWFQ bandwidth provisioning a better choice.

What you're asking, to start shaping at 75% doesn't make sense since there's no congestion, unless, as above, we keep some traffic from exceeding 75%.

If you could further describe what you're trying to accomplish. Someone might be able to provide the MQC sample that would provide it.